I used to use it just for the ability to individually stage chunks/lines, because I find "git add -p" incredibly hard to use. Now I tend to use GitX-dev for OSX [1] to do that instead, because I find it more powerful than Github for Mac. There's also a ncurses-based interface for git called tig [2] which can also do the chunk staging, but I only use it when I don't have a GUI.
[1] http://rowanj.github.io/gitx/ [2] http://jonas.nitro.dk/tig/