The Key To The Abode of Peace
Al-I'Tibaa'A and The Principles of Fiqh Of The Righteous Predecessors
A Description of the Supererogatory (Sunnah) Prayers
Shaykh Saalih al-Fawzaan said, "O Muslim, be consistent with the regular supererogatory Prayers, imitating the excellent example of the Prophet (peace be upon him). Being Consistent with the supererogatory Prayer is also a way to compensate for any fault or deficiency that transpired in the obligatory Prayers.
Man is prone to fall short and fail to achieve perfection, and therefore he is in need of something to compensate for his negligence.
For this reason - O Muslim - you cannot afford to neglect the regular supererogatory Prayers, as this will earn you extra rewards, which you will find with your Lord."