The reason why fasting is prescribed
Question

What is the reason why fasting is prescribed?.
Praise be to Allah.Firstly we must note that one of the names of Allah is al-Hakeem (the Most Wise). The word Hakeem is derived from the same root as hukm (ruling) and hikmah (wisdom). Allah alone is the One Who issues rulings, and His rulings are the most wise and perfect. 
Secondly: 
Allah does not prescribe any ruling but there is great wisdom behind it, which we may understand, or our minds may not be guided to understand it. We may know some of it but a great deal is hidden from us. 
Thirdly: 
Allah has mentioned the reason and wisdom behind His enjoining of fasting upon us, as He says (interpretation of the meaning): 
“O you who believe! Observing As-Sawm (the fasting) is prescribed for you as it was prescribed for those before you, that you may become Al-Muttaqoon (the pious)” [Al-Baqarah 2:183]
Fasting is a means of attaining taqwa (piety, being conscious of Allah), and taqwa means doing that which Allah has enjoined and avoiding that which He has forbidden. 
Fasting is one of the greatest means of helping a person to fulfil the commands of Islam.
 The scholars (may Allah have mercy on them) have mentioned some of the reasons why fasting is prescribed, all of which are characteristics of taqwa, but there is nothing wrong with quoting them here, to draw the attention of fasting people to them and make them keen to attain them. 
Among the reasons behind fasting are: 
1 – Fasting is a means that makes us appreciate and give thanks for pleasures. For fasting means giving up eating, drinking and intercourse, which are among the greatest pleasures. By giving them up for a short time, we begin to appreciate their value. Because the blessings of Allah are not recognized, but when you abstain from them, you begin to recognize them, so this motivates you to be grateful for them.
 2 – Fasting is a means of giving up haraam things, because if a person can give up halaal things in order to please Allah and for fear of His painful torment, then he will be more likely to refrain from haraam things. So fasting is a means of avoiding the things that Allah has forbidden. 
3 – Fasting enables us to control our desires, because when a person is full his desires grow, but if he is hungry then his desire becomes weak. Hence the Prophet (peace and blessings of Allah be upon him) said: “O young men! Whoever among you can afford to get married, let him do so, for it is more effective in lowering the gaze and protecting one’s chastity. Whoever cannot do that, let him fast, for it will be a shield for him.” 
4 – Fasting makes us feel compassion and empathy towards the poor, because when the fasting person tastes the pain of hunger for a while, he remembers those who are in this situation all the time, so he will hasten to do acts of kindness to them and show compassion towards them. So fasting is a means of feeling empathy with the poor.
 5 – Fasting humiliates and weakens the Shaytaan; it weakens the effects of his whispers (waswaas) on a person and reduces his sins. That is because the Shaytaan “flows through the son of Adam like blood” as the Prophet (peace and blessings of Allah be upon him) said, but fasting narrows the passages through which the Shaytaan flows, so his influence grows less. 
Shaykh al-Islam said in Majmoo’ al-Fatawa, 25/246 
Undoubtedly blood is created from food and drink, so when a person eats and drinks, the passages through which the devils flow – which is the blood – become wide. But if a person fasts, the passages through which the devils flow become narrow, so hearts are motivated to do good deeds, and to give up evil deeds. 
6 – The fasting person is training himself to remember that Allah is always watching, so he gives up the things that he desires even though he is able to take them, because he knows that Allah can see him. 
7 – Fasting means developing an attitude of asceticism towards this world and its desires, and seeking that which is with Allah. 
8 – It makes the Muslim get used to doing a great deal of acts of worship, because the fasting person usually does more acts of worship and gets used to that.  
These are some of the reasons why fasting is enjoined. We ask Allah to help us to achieve them and to worship Him properly. 
And Allah knows best. 
See Tafseer al-Sa’di, p. 116; Ibn al-Qayyim’s footnotes on al-Rawd al-Murabba’, 3/344; al-Mawsoo’ah al-Fiqhiyyah, 28/9.