Well, the upsides of forum-RPs is they're less prone to scheduling issues than in-person or instant-messages, and unlike RP via E-mail, far more conductive to people joining mid-session. Or just sharing in some insane actions one of the players took. Albeit not as dramatic as a livestream.

Being able to stumble upon and dogpile into an RP while it's mid-game is actually a pretty huge thing. And that aspect is kinda laid-back like roaming a convention and just inviting yourself to a table that looks neat.

That's the basics of the forum-medium.