I feel like SourceTree actually only maps every Git command with a button, nothing more. Contrast this with Github for Windows/Github for Mac. These applications are trying to make getting startet with Git easy. If you're using Git with a GUI, that's the way to go, not with a toolbar full of obscure buttons.
Once you're experienced and you need to use all the "advanced" features of git, SourceTree makes it easier to do so without having to remember every correct syntax without the fear of doing something fatal to your working commit (before pushing it).
EDIT: Not to mention, the integration with git-flow is just amazing for someone who wants a set work flow (pun not intended) and naming convention without the extra overhead of thinking about how to manage it.
EDIT2: Typos and grammar.