Well, the discussion about playing the game itself happens in GnR, so there really isn't any need to have that discussion anywhere else. However, having an outside place (away from this website) to talk about players is preferred, because drama is always best viewed from the outside (or something like that). They don't have to worry about osu mods tracking people down for talking shit about other people because their reddit accounts aren't connected to their osu accounts.
At least that's what I think.
If you want my opinion, I would avoid that subreddit like the plague. No offense towards anyone that hangs there, of course. You have to admit, though, that place spawns nothing but discussion about drama.