What Is the Wisdom of Zakah?
Question

Is there any particular reason why zakah is prescribed?
Praise be to Allah.Firstly: 
It should be noted that Allah does not prescribe anything that is not for the best reasons and does not achieve the best interests. Allah is the All-Knowing, Who encompasses all things by His knowledge, and He is the All-Wise Who does not prescribe anything except for a reason. 
Secondly: 
Wisdom of Zakah
With regard to the reason behind the prescription of zakah , the scholars have mentioned many reasons for it, including the following: 
1 – To complete and perfect a person’s Islam, because it is one of the pillars of Islam. If a person fulfils it, his Islam is complete. Undoubtedly this is an important aim for every Muslim, and every believing Muslim will strive to perfect his faith. 
2 – It is indicative of a person’s sincerity of faith, because wealth is dear to people, and something that is loved will not be given up except for something that is equally or more loved, or rather for something that is more loved. Hence it is called sadaqah, because it is indicative of the sincerity (sidq) of a person’s desire to please Allah, may He be exalted. 
3 – It cleanses the character of the one who gives it, so that he is saved from being a miser and is included among the generous, because if he accustoms himself to giving, whether it is giving knowledge or giving wealth or using his position to help others, that giving will become a characteristic for him and will be part of his nature, until he gets upset if there is a day when he does not give. Like the hunter who gets used to hunting, if one day he does not hunt, you will find him feeling anxious. Similarly the one who has gotten used to being generous will feel anxious if he does not give some of his wealth or use his position to help others one day. 
4 – It gladdens the heart. If a person gives something, especially wealth, he feels a sense of joy. This is something that is tried and tested, but it is subject to the condition that he gives gladly and willingly, not grudgingly. 
Ibn al-Qayyim said in Zaad al-Ma’aad that giving and generosity are means of gladdening the heart, but no one benefits from that except the one who gives gladly and willingly, and gives the wealth from his heart before he gives it from his hand. As for the one who gives from his hand but his heart is still attached to the wealth, his heart, he will never benefit from that giving. 
5 – It joins a person to the ranks of the true believers. “None of you truly believes until he loves for his brother what he loves for himself.” Just as you love to be given money with which you can meet your needs, you should also love to give it to your brother, thereby you will perfect your faith. 
6 – It is one of the means of entering Paradise, because Paradise is for the one who “speaks good words, spreads (the greeting of) salaam, feeds the hungry and prays at night when the people are asleep.” And all of us are striving to enter Paradise. 
7 – It makes the Muslim society like a single family, in which those who have the means show compassion to those who do not have the means, and the rich to those who are in hardship. Thus a person feels that he has brothers to whom he must do good as Allah has been good to him. Allah says (interpretation of the meaning): 
“and do good as Allah has been good to you” [al-Qasas 28:77]
So the Muslim ummah becomes like a single family. This is what is known nowadays as “social security”, but zakah is better than that, because a person is paying it as an obligatory duty and benefitting his brothers thereby. 
8 – It extinguishes the flames of revolution among the poor, for a poor man may be provoked by finding a man who can ride whatever kind of vehicle he wants, and live in whatever kind of palace he wants, and eat whatever kind of food he wants, whereas he has no means of transportation but his own two feet, and can sleep only on the ground, and so on, so undoubtedly he will feel some kind of resentment in his heart. 
But if the rich are generous to the poor , they will prevent rebellion and soothe their anger and they will say: we have brothers who acknowledge us at times of hardship, so they will feel love towards the rich.
 9 – It prevents financial crimes such as robbery and stealing and the like, because the poor will get enough to meet their needs, and they will excuse the rich because they are giving them some of their wealth, and they will see that they are treating them well, so they will not transgress against them. 
10 – Salvation from the heat of the Day of Resurrection. The Prophet (peace and blessings of Allah be upon him) said: “Every man will be in the shade of his charity on the Day of Resurrection.” Classed as saheeh by al-Albaani in Saheeh al-Jaami’ (4510). And he listed among those whom Allah will shade with His shade on the Day when there will be no shade but His: “… a man who gives charity and conceals it so much that his left hand does not know what his right hand is doing.” Agreed upon. 
11 – It leads a man to learn about the laws of Allah, because he can never pay his zakah until he has learned the rulings on zakah, the kinds of wealth that are subject to zakah and those who are entitled to receive it, and other information that he needs. 
12 – It purifies wealth, so that the wealth will grow literally and metaphorically. If a person gives charity from his wealth, that this will protect him from problems, and Allah may increase his provision because of his giving charity. Hence it says in the hadeeth: “Wealth never decreases because of charity.” Narrated by Muslim, 2588. This is something that is tried and tested, because a miser’s wealth may be overtaken by something that destroys it or most of it, such as fire, and great losses, or sickness that forces him to seek treatment which takes up much of his wealth. 
13 – It is a means of bringing down blessings. In the hadeeth it says: “No people ever withhold the zakah of their wealth but rain is withheld from the sky.” Classed as saheeh by al-Albaani in Saheeh al-Jaami’, 5204. 
14 – “Charity given in secret extinguishes the wrath of the Lord,” as was narrated from the Messenger (peace and blessings of Allah be upon him) and classed as saheeh by al-Albaani in Saheeh al-Jaami’, 3759. 
15 – It wards off a bad death.
16 – It wrestles with the calamity that comes down from heaven and prevents it reaching earth. 
17 – It expiates for sins. The Messenger (peace and blessings of Allah be upon him) said: “Charity extinguishes sins as water extinguishes fire.” Classed as saheeh by al-Albaani in Saheeh al-Jaami’, 5136.  
Reference:
Al-Sharh al-Mumti’ by Ibn `Uthaymeen, 6/4-7.