Looper (2012) – Why do the Loops have to be closed?

Just watched Looper for the first time, thoroughly enjoyed it, but one question kept nagging at me, why does the loop need to be closed? Or do they even all get closed? Since the Rainmaker "started closing all the loops", does that then mean that most loops werent closed before he came into power? If so, what would necessitate a loop being closed?

So yes, I get the movie tells me right at the beginning, that time travel is highly illegal, so they close the loop so the loopers cant tell the authorities, but why 30 years? Why not just then and there, they have 30 years to snitch? The loopers dont really know anything about the organization, they just show up at an undisclosed location and 1 taps a faceless someone, they don't know anything of value to the authorities anyway?

And also the movie, felt to me, like it said that all loops need to be closed at some point, and thats what the loopers sign up for when they take the job, but then why is the Rainmaker bad for "closing all the loops" if they were already all being closed?

Idk it just seemed overly complicated, and im 100% sure theres a very simple answer in there that I just missed, but please if someone could tell me, because it did really confuse me.