Shah Wali’ullaah Rahimahullaah has stated the following six benefits of fasting:

1.) Fasting is a great act of virtue by which the angelic qualities of a person are strengthened and animalistic qualities are weakened. There is nothing more effective than fasting in polishing the face of one’s spirituality and overpowering the Nafs. The following Hadith makes it evident that fasting is a great virtue: Rasulullaah SalAllaahu Alaihi Wasallam said, “While the rewards for every deed is multiplied from ten up to seven hundred times, Allaah says, ‘Except for fasting because fasting is for Me and I shall reward it personally since a person forsakes his desires and eating solely for Me.’

The fasting person shall have two moments of joy; one when he terminates the fast and one when he meets his Rabb (on the Day of Qiyaamah and receives his reward).” (Bukhaari no. 7492)

While the risk of ostentation and show exists in every good deed, it is not found in fasting because fasting is a secretive act, known only to Allaah. In fact, fasting is such a great act that even the angels do not know how much rewards to record for it. The standard for conferring rewards does not apply to fasting and its rewards will be conferred by Allaah Himself on the Day of Qiyaamah.

2.) Fasting secures forgiveness for sins just as effectively as it quells the drive of bestiality within. Rasulullaah SalAllaahu Alaihi Wasallam said, that when a person fasts during Ramadhaan with Imaan and with the hope of being rewarded, all his previous sins will be forgiven. (Bukhaari no. 1901, Muslim no. 760)

Ahaadeeth also state the same effect for the Taraaweeh salaah and for performing Nafl salaah during the night of Qadr. 

3.) A person develops a deep resemblance with the angels by fasting and the more he resembles them, the more they love him. Rasulullaah SalAllaahu Alaihi Wasallam stated that the smell form the mouth of a fasting person (because of his empty belly) is more beloved to Allaah than the smell of musk. (Bukhaari no. 7538)

When Allaah then loves the fasting person, the angels will naturally follow suit.

4.) Amongst the three factors which pose an obstacle to attaining salvation is the veil of customary practices. When fasting is observed with all its due right, and it becomes second nature to people, it will safeguard them against the evils of customary rituals. Rasulullaah SalAllaahu Alaihi Wasallam said, “When any of you is fasting, he should, abstain from nonsensical and vulgar talks and should not be noisy. If someone else then wishes to cast verbal abuse at him or argue with him, he should excuse himself saying, ‘I am fasting’.” (Bukhaari no. 1904, Muslim no. 1151)

5.) When a community collectively gives importance to fasting, the rebellious ones amongst them will become shackled, the doors of Jannah will be flung open for them, and the gates of Jahannam will be closed for them. (Bukhaari no. 1899, Muslim no. 1079)

NOTE: Because the devout Muslims are involved in good deeds during the month of Ramadhaan, the common Muslims are also more inclined towards good deeds. Now since the rewards for good deeds are multiplied during Ramadhaan, the doors of Jannah are flung open for them. They are also less inclined towards sin, because of which the gates of Jahannam are closed to them. All those Muslims worthy of being called Muslims are affected by this atmosphere of good and the Shayaateen from amongst man and Jinn find it impossible to mislead them. They therefore become effectively shackled in their pursuits to misguide such Muslims. All of this applies to the people of Imaan who are inclined towards the blessings and virtues of Ramadhaan and not to the Kuffaar and people who are blind to the greatness of Ramadhaan.

6.) The fasting person attaches himself to Allaah. To understand this, we need to look at the Hadith in which Allaah says, “Fasting is for Me and I shall reward it.” Here the words are recited as أنا أَجْزِي بِه  (“I shall reward it”). This is the proper manner of reciting the Hadith according to the context. However, according to the Sufis, the Hadith can be read as أنا أُجْزى به , meaning “I shall be the reward for it.” This meaning exposes this benefit of fasting.            

This happens because when a person makes an effort to suppress his bestial instincts and to curb the evil within himself, he is really purifying his soul. As a result, all his deeds also become pure and he joins the sanctified ranks of those who are close to Allaah. His knowledge of the unseen then also manifests itself to a greater degree because his perceptions become stronger. In this manner, he is able to attach himself to Allaah. (Extracted from Hujjatullaahi al- Baalighah with commentary – Rahmatullaah al-Waasi’ah)