If you´re provisioning for peak usage, by definition you´re wasting money at all other times. Even worse, when your marketing department pulls off a coup, and they don´t tell engineering, your systems are vulnerable to being overwhelmed. By implementing auto-scaling, we ensure that you´re always using the right number of resources (and if your traffic blows up, you´ll be ready).
Some other benefits of auto-scaling are that it lets you provision spot instances across availability zones, improving your cost and your reliability.