Earlier quoted context omitted.
> what can you do with magit that you can't do with those? It's not the "what", it's the "how". Magit enables extremely fancy workflows with great simplicity, and its UI is self-discoverable and well laid-out. Magit has the best UX of any git tool.
> its UI is self-discoverable and well laid-out. Magit has the best UX of any git tool. Which others have you tried? The way you described Magit is exactly how I would describe SmartGit and also the built-in git features of several IDEs, including the Jetbrains family and Visual Studio (although I dislike Visual Studio because it's slow).
For me, Magit code staging is the best I've tried: staging whole files is as easy as staging single lines, which means my commits are more meaningful. Magit also allows me to use some fancy features without having to remember they exist, thanks to the well-thought command popups. Last, it's integrated with Emacs: this means I can move through code and write my commit messages in a familiar, coherent way.
The final advantage for me is that Emacs and Magit are forever. They will live longer than any other tool out there.