This is based on my experiences with Magit, so maybe this doesn't work quite the same with SublimeGit, but the thing I find most useful is the ability to select exactly which lines to stage in a commit. You can do this on the command-line with git add -p, but it's much easier and much more natural to do it in the text editor itself. Once you get used to doing this, it's hard to imagine going back.

> is it just using sublime as a GUI for git?

Basically just that, but with shortcuts for stuff like stage current file, commit, push, etc