Oooh...open can, extract worms, stand back and watch
.
Seriously, there's a lot of info around on this topic. I don't know if it's been covered on here before, but
http://www.archersreference.pwp.blueyonder.co.uk/ has one of the better, clearer explanations and is free to download.
The short answer is that your button is one of the more critcal bits of your equipment. Because its main function is to allow the arrow to fly straight and stable as soon as possible after leaving the bow (at least I think that's what it does...someone please tell me if I'm wrong), it's basically responsible (within the limits of your shooting) for how cleanly your arrows leave the bow and fly.