Why do we have supererogatory worship?
There is a Hadith in Sahih Bukhari:
Translation: My slave comes closer to me with no act more beloved to me than the obligatory acts of worship (Fardh) which I have assigned to him and my slave keeps coming closer to me with supererogatory acts of worship (Nafl), so much so that I start loving him. When I make him my beloved, then I become his ears with whom he hears. I become his eyes with which he sees. I become his hand with which he holds and I become his legs with which he walks. If he asks me for something, I surely grant it. If he asks me for protection, I surely grant it. If I want to do something, I don’t hesitate the way I wait before taking the soul of the faithful when does not desire it and I don’t like hurting him. (Sahih Bukhari, Hadith No. 6502)
Apart from Obligatory Salaat, Sunnah and supererogatory Salaat have been decreed for various times of the day:
(1) Before the Obligatory Salaat.
(2) After the Obligatory Salaat.
(3) At other times like Salaat Ul Ishraaq after dawn, Salaat Ul Chaasht after the sun rises over head, Salaat Ul Awwabeen in the early part of the night, Salaat Ul Tahajjud in the middle or last part of the night.
The benefit of Sunnah and supererogatory Salaat before the Obligatory Salaat is that one remains busy in the various affairs of the world. Usually, all those other engagements make one heedless of the remembrance of Almighty Allah and become impediments to it. This in turn allows the desires to raise their head. When these take root, the angelic faculty is suppressed by them. Thus, because of being busy in worldly affairs, one is ensnared by these desires. This in turn affects the heart and the soul and darkens them.
For this reason, Sunnah and supererogatory Salaat have been decreed which cleanse the heart and the soul and increase one’s concentration and devotion.
It is mentioned in Al Bahr Ur Raaiq Sharh Kanz Ud Daqaaiq:
Translation: The Sunnah have been decreed to complete the obligatory forms of worship.