It leverages 'orchestrate' (which is what gulp uses) to do deploys across a variety of hosts, keeping the old versions around should you need to switch back easily.
But why wouldn't I just use Gulp then?
gulp doesn't define groups of servers, or connect to them over encrypted sockets. This does.
FYI the authors (which I'm not) seem to be gulp fans too, and want to leverage as much as they can.
Try deploying just with grunt and you will see the difference.
The grunt-aws module makes deploying to S3 and cloudfront a breeze. We switched to this for all our FE devs awhile ago and it's been painless.