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."