وحكمه: أن يثاب المرء على فعله ولا يعاقب بتركه
Performing nafl brings reward, while omitting it does not incur punishment.
Both elements are necessary. The first shows that voluntary worship is genuinely commendable; the second shows that leaving it is not a breach equivalent to omitting fard or wajib.
A person who regularly observes required worship may offer extra prayers and gain reward. If an extra prayer is not performed on a given day, its omission does not acquire the legal consequence of neglecting a binding duty.