Grand slams and Masters 1000 tournaments are mandatory and the fact that if you are allowed to enter them and you don't, you get zero point and can't replace it for another result, is the absolute key of that mandatory character. There's still a flexibility that actually players can indeed decide to skip them if they really want, but anyway, they will get those zero points and not replace them, which guarantees that this system still keeps being "mandatory" in a way.
And I think this mandatory character (which top-players asked for during the US Open 1988) is an absolute key of the visibility and quality of the Tour. In the 70s and 80s when it didn't exist, top-players could choose whatever tournament they wanted, especially for money reasons, and it's one of the main reasons why top-players seldom played against each other and why the rankings were not reliable and not transparent in that time (also the fact that the rankings were calculated with an "average" from less tournaments and the use of the bonus for wins against highly ranked players).
They tried to make something similar for ATP500 tournaments because they asked those tournaments a lot, in terms of increasing the prize money especially (they didn't ask so much to ATP250 tournaments hence the no-obligation there), then they needed to guarantee them some top-players participation, but it's a far less strict obligation.
I know that.