All open source fanatics may know that GitHub houses some of the best projects out there, like jQuery or HTML5 Boilerplate, two projects I actively clone. A while back last year, they released their Mac application to make Git easier to use, even though Mac users do have applications like Tower already. And now, that Windows is still the most popular desktop operating system out there, GitHub wanted to support all Windows users who would like to use Git (even though Git by itself isn't too perfect on Windows) with a fancy Metro-like UI.
Although I will say, as the Mac version was a bit glitchy (maybe I should update :P), I hope the Windows version has no major bugs. But maybe GutHub fixed these issues.
Anyways, all you Git users on Windows are probably very happy to finally get a nice looking client. As for myself, I will stick with my favorite, Cornerstone 2.