If you do want to see why an interface would be needed, you may want to have a look at Play By Web for Space Empires IV. When the server is back Online that is.
PBW ran *a lot* of games (somewhere above a hundred, likely two hundred and some) from two to twenty players, with different settings and even mods were featured, along with all the features Gandalph mentioned.
I do not know what were the specifications of the server though, so I cannot say how much is needed. And don't ask me either how PBW is working.
But it works very well, while you would have a problem to handle a hundred of games without an interface.