والنفل: عبارة عن الزيادة
Linguistically, nafl means an addition or something extra.
The same word can describe an additional share or grant beyond a person's principal portion of spoils. Its use in worship follows the idea of an act performed beyond what is already required.
Being additional does not make the devotion worthless. It refers to an act offered in pursuit of Allah's pleasure beyond the core requirement of fard and wajib. The value of an act and whether its performance is binding are separate questions.