It has many capabilities useful to the modern web dev workflow.
Of most note to me, it acts as a "project creation" tool. AKA "A Scaffolding generator."
It will pull things like HTML5 Boilerplate, jQuery, Backbone.js, etc down from github, and properly generate the project files you need to start a project with those dependencies.
You dont have to worry about how any of it fits together, it will get you up and running with the latest version of everything with a simple commandline.
It also does things like minify css and javascript, as well as compiling LESS/SASS and Coffeescript.
They are unfortunately trying to make a single tool that solves many problems, when they might be better served by making many tools, that are all good at 1 thing each... I'm still waiting for them to launch this so I can see how that all pans out.