Ah yes, we actually have a small community on the Phansite discord where a fair number of us like to do a bit of planning beforehand. There are like 4 or 5 people who participate frequently and a few others who don't quite have as much free time.
Why is uh.. A weird question. Its kinda just because we want to, y'know? Its a way to hang out that's kinda like putting on a play. So I'm glad to see ya've liked a few of 'em.
It started in the primordial chaos of the early phansite, where we didn't have tags so people would pretend to be a character in some threads while others are just meming for fun, that sort of thing.
Wasn't really a single person coming up with the idea, though the formalization of the practice is often attributed to the user Yu Narukami, who made Awakening-type threads that were some of the first RP-focused threads.