For the same reason that multitrack tape decks have multiple outs -- for sending to a mixer for signal routing and tweaking on its way to the mastering deck. Now so much can be done on the computer -- mixing, effects processing, mastering -- that it seems weird to have so many outs. But that wasn't the case just a few years ago. And in real studios, it still isn't -- many folks still prefer to apply processing with outboard boxes, not plug-ins.