There are many benefits of Nafil and Mustahab acts of worship. A few are listed below:
1-Sunnah and Nafil acts bring one closer to Allah and help in inculcating His love. They also make one the beloved of Allah and all his duas will be accepted and he is granted Allah's protection.
A Hadith e Qudsi says,"...My servant continues to draw nearer to Me with supererogatory worship (nawafil) so that I shall love him. When I love him, I shall be his hearing with which he shall hear, his sight with which he shall see, his hands with which he shall hold, and his feet with which he shall walk. And if he asks (something) of Me, I shall surely give it to him, and if he takes refuge in Me, I shall certainly grant him it.’”
2-Mustahabat act as a prevention against sin.
One who is regular in Tahajjud, Ishraq, Chasht and Aw-wabeen (Nafil salaahs) will abstain from sin to a greater degree than the one who performs only the five Fardh Salaat.
Furthermore, a regular performer of Tahajjud, etc. is considered a pious person by others. This consideration in itself acts as an impediment to sin, since such a person will feel ashamed to commit sins.
3-Mustahab act may lead to najaat (salvation).
Another benefit of Mustahabat is that sometimes Allah Taala is pleased so much with a certain Mustahab act that najaat (salvation) is obtained by virtue of it. There are various incidents related in reliable Hadith books to substantiate it (such as the sinful woman of Bani Israel who was granted maghfirah due to giving water to a thirsty dog).
[Adapted from a speech by Shaikh Maseehullah Khan (R.A.)]