If you prefer to do most of your development inside the terminal, changing windows to move to a GUI git client can be annoying. Since tig runs inside the terminal, that's avoided.
Since you have to get used to working in the terminal (if you are not already used to doing that) and learn the commands, it has a steeper learning curve than GUI clients which are usually more intuitive and easier to grasp.
Since it's integrated with Emacs, diffs are very easy to fix. You can jump right to any file you want to fix as soon as it comes up in the logs or in the status view.
Magit is only useful if your text editor of choice is Emacs. It wouldn't really make any sense to open up emacs just to run Magit if you use another editor.
Although there's autocompletion for commands, it still requires recalling at least the beginning of a command while all that's required using a GUI is recognizing the desired outcome.