Basically you get a bunch of fighters together (12 in this case) and get them to punch, kick, bite and thump the bejesus out of each other then you're close.
It's hard to say just why 2D fighting crossover games exist. Well, actually, I think I know why they exist (hint: it starts with a $ and ends with another $).
I love Street Fighter, always have, even back when the occasional fireball was more a freak product of luck that elicited looks of shock than a planned attack. But I'm not that big a fan of the Street Fighter Anniversary Collection.
It takes a lot to make a Nintendo game seem bizarre, probably because no matter how strange a Gamecube game is, players are having so much fun playing it they don't notice.