not basically lets you pivot between logically equivalent renderings of the same thing. DeMorgan’s law says that:
a > b == not(a <= b)
and this extends to compound propositions like:
(a > b or a <= c) == not (a <= b and a > c)
Depending on what your function is doing, it might be easier to think of the logic in terms of or and having a choice between two things, or it might be easier to think of it in terms of and where several things all have to be true.
By supporting not you can turn an and into an or by pulling out a not to the front and vice versa.
