To put a discussion of simulationism aside (maybe it should be moved to another thread?), I have this theory that I've jokingly called it Super Theory of Super Everything, and I keep postponing putting it to paper.
So.
There's an infinite set E, that contains everything, everything, Super Everything possible in a roleplaying game, from defeating princesses to rescuing dragons to finding love to getting shot in the face to randomly dying from complications of teeth cavities. If you can think of it, it is included in E.
There's an infinite set S that contains everything possible in a particular system. Let's suppose there's an event o, o ∈ S, but o ∉ E. So, the system allows for a thing that is not possible in roleplaying game. Yeah, impossible, so S⊂E, or, in other words, a system cannot grant you any more freedom that you already have, only take it away. Chat calls it Loverdrive Theorem, and I am egotistical enough to accept such name.
Why do RPGs have rules then?
There's a subset G of E, that includes everything that you know you do want. Things that you are excited about. You can just, like, have those things. Introduce them directly, will them into existence: that's how ERP works. You don't need no rules for that, but it is possible for G\S to exist, things that you do want, but are not possible within the system. In mathematic terms, such scenario is called "this sucks on ice", and if G⊂S, it's called "woohoo, we have the bare minimum, who cares?".
Why do RPGs have rules then?
There's a subset B of E, things that you don't want. You also can, like, just not have those things, will them out of existence: that's how ERP works. But, unlike G, there's a good scenario. When B⊄S, when the system doesn't allow for things you don't want, great! You don't have to worry about them and can pursue G without a care in the world.
Why do RPGs have the rules then? To exclude B. To remove a need to care about #### you don't want so you can chase the high of G. Seek for a G-spot, if you will.