Under basic Part 91 rules for certified aircraft, everything aboard has to be documented and working. The aircraft can remain airworthy if failed equipment isn’t required for the operation, and is placarded and isolated from other systems. But an airplane is like any other mechanical contrivance: it’s subject to wear and tear: A system may function, but not as intended.
A good example can be braking systems. Those on typical personal airplanes are hydraulically actuated, as is the case with modern automobiles. Almost anyone who’s owned a car knows its brake system can need maintenance but still work, sort of, and the same is true for an airplane’s brake system. It’s a maintenance-related deficiency and often gets deferred. That’s fine on a dry, calm day but becomes riskier on a windy, icy, wintry one.
