To answer the question, we need first to consider the notion of necessity and the related notion of contingency. These are so-called "modal" notions. Other modal notions include those of possibility, impossibility, non-necessity, and noncontingency. All play a crucial role in philosophical thinking about matters to do with logic, metaphysics, morality, law, etc. This is because none of these modal notions is univocal in meaning. There are, so to speak, different "species" of the generic notions of necessity, contingency, possibility, and the rest.