Why Do We Need Prophets?
Question

Why do people need the prophets?
Praise be to Allah.Who are the prophets?
Prophets are the Messengers of Allah to His slaves; they convey His commands, and give them glad tidings of the delights that Allah has promised them if they obey His commands, and they warn them of the abiding punishment if they go against His prohibitions. They tell them the stories of the past nations and the punishment and vengeance that befell them in this world because they disobeyed the commands of their Lord. 
These divine commands and prohibitions cannot be known through independent thinking, hence Allah has prescribed laws and enjoined commands and prohibitions, to honor mankind and protect their interests, because people may follow their desires and thus transgress the limits and abuse people and deprive them of their rights. 
Why Did Allah Send Prophets and Messengers?
So by His wisdom Allah sent among them from time to time messengers to remind them of the commands of Allah and to warn them against falling into sin, to preach to them and to tell them the stories of those who came before them. For when people hear wondrous stories it makes their minds alert, so they understand and increase in knowledge and understand correctly. For the more people hear, the more ideas they will have; the more ideas they have, the more they will think; the more they think, the more they will know; and the more they know, the more they will do.  So there is no alternative to sending messengers in order to explain the truth. (A’lam al-Nubuwwah by ‘Ali ibn Muhammad al-Mawardi, p. 33) 
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) – Ahmad ibn ‘Abd al-Halim ibn ‘Abd al-Salam, who is well known as Ibn Taymiyah, born in 661 AH and died in 728 AH, one of the greatest scholars of Islam who wrote many valuable books – said: 
“Messengership is essential to guide mankind to that which is best for them in this world and in the Hereafter. Man cannot follow what is best for him with regard to the Hereafter unless he follows the message and he cannot be guided to what is best for him in this world unless he follows the message. Man needs the Shari`ah (Divine law) because he has two motives, to bring that which will benefit him and to ward off that which will harm him. This Shari`ah is the light of Allah on this earth, and His justice among His slaves, and the refuge which whoever enters it will be safe. 
Shari`ah does not mean distinguishing between what is beneficial and what is harmful on a physical basis, because even animals are able to do this. Donkeys and camels are able to differentiate between barley and dust. Rather the distinction is between deeds which will harm a person in this world and the Hereafter, and deeds which will benefit him in this world and in the Hereafter, such as faith, Tawhid, justice, righteousness, kindness, trustworthiness, chastity, courage, knowledge, patience, enjoining what is good and forbidding what is evil, upholding the ties of kinship, honouring one’s parents, treating neighbours well, recognizing people’s rights, sincerely doing things for the sake of Allah, putting one's trust in Him, seeking His help, accepting His decree, submitting to His will, believing in Him and in His messengers in all that they have told us, and other deeds which are of benefit to a person in this world and in the Hereafter. The opposite of that leads him to misery and doom in this world and in the Hereafter. 
Were it not for the messengers, our minds could not guide us to differentiate between the beneficial and the harmful in this life in a detailed manner. One of the greatest blessings that Allah has bestowed upon His slaves is that He sent messengers to them and revealed books to them, and showed them the Straight Path. Were it not for that, they would have been like cattle, and even worse off. So whoever accepts the Message of Allah and adheres to it is one of the best of mankind, and whoever rejects it and ignores it is one of the worst of mankind, even worse off than dogs and pigs, and the vilest of the vile. The people of this world could not survive except by virtue of the messengers’ teachings, traces of which are still extant amongst them. If these traces of the messengers vanished from the earth and their teachings were wiped out, Allah would destroy the upper and lower realms and the Hour would begin. 
The need of the people of the earth for the messengers is not like their need for the sun, moon, wind and rain, or like a man’s need for his life, or like the eye’s need for light, or like the body’s need for food and drink. Rather it is greater than that, greater than his need for anything you could think of. The messengers (peace and blessings of Allah be upon them) are intermediaries between Allah and His creation, conveying His commands and prohibitions. They are ambassadors from Him to His slaves. The last and greatest of them, the noblest before his Lord, was Muhammad (peace and blessings of Allah be upon him, and upon all of them). Allah sent him as a mercy to the worlds, guidance for those who want to draw closer to Allah, proof which left no excuse for all people. He enjoined the people to obey him, love him, respect him, support him, and acknowledge his rights. Allah took the covenant from all the messengers and prophets that they would believe in him and follow him, and He commanded them to take the same covenant from the believers who followed them. He sent him just before the Hour as a bringer of glad tidings and a warner, calling people to Allah by His leave and as a brightly shining lamp. He concluded the line of messengers with him. Through him He guided people and dispelled misguidance, taught them and dispelled ignorance. Through his message He opened blind eyes, deaf ears and hard hearts. Through his message He filled the world with light after it had been in darkness, He brought people together after they had been divided, He straightened the crooked path of mankind and showed them the clear way. He opened his heart for him and removed from him his burden, and raised high his fame (cf. al-Sharh 94:1-3). He inflicted humiliation and shame on those who went against his command. He sent him (peace and blessings of Allah be upon him) at a time when there had been no messengers for a long while and when the Books had vanished, when words had been distorted and laws changed, when all peoples referred to their own unfair opinions, developed their own ideas about Allah and judged among people by their own corrupt ideas and whims and desires. Through him Allah guided mankind and clarified the different means of drawing closer to Allah. Through him, He brought the people forth from darkness into light. Through him, He differentiated between those who will prosper and those who are immoral. So whoever follows his guidance is truly guided, and whoever turns away from his path is misguided and has deviated. May Allah send blessings and peace upon him and upon all the messengers and prophets . (( Wujub al-I’tisam bi’l-Risalah by Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him), vol. 19, p. 99-102; and from Majmu’ al-Fatawa. See Lawami’ al-Anwar al-Bahiyyah, vol. 2, p. 216, 236) 
Why do we need prophets?
We may sum up man’s need for the messengers as follows: 

Man is created and subjugated, and he has to know his Creator and what He wants of him, and why he was created. Man cannot come to know that independently. He has no way of finding that out except through knowing the prophets and messengers , and the guidance and light which they brought.
Man is composed of body and soul, His body is nourished by his food and drink, but the nourishment of his soul is that which was explained by its Creator, namely the true religion and righteous deeds. The prophets and messengers brought the true religion and taught them to do righteous deeds.
Man is religious by nature; he has to have a religion to follow, and this religion has to be correct. There is no way to the true religion except through believing in the prophets and messengers and believing in the message they brought.
Man needs the way through which he may attain the pleasure of Allah in this world, and reach His Paradise and bliss in the Hereafter. No one can show these ways and guide people to them apart from the prophets and messengers.
Man is weak by nature, and there are many enemies lying in wait for him, such as the Shaytan who wants to lead him astray and bad companions who make evil things attractive to him, and his own self which is inclined towards evil. Hence he needs something to protect himself from the plots of his enemies. The prophets and messengers guide him towards that and show it to him clearly.
Man is sociable by nature. In human meetings and interactions, there have to be laws to guide people in a fair and just manner – otherwise the law of the jungle would prevail. This guiding law must protect the rights of all those who have rights, without neglecting or exaggerating in any way. No one can produce a perfect law except the prophets and messengers.
Man needs something that will give him security and peace of mind, and guide him to the means of true happiness. This is what the prophets and messengers guide people towards.

And Allah knows best.