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.
Steeper barrier to entry as compared to a GUI solution
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.