Besides what others have said about GUI creation, Tk is a different kind of programming, involving an "event loop". You need to think abit differently when programming in event loops. But after you get used to it, there is alot more you can do, like setup background timers, multi-tasking, etc.

POE is another example of "event loop" programming, and you can even use Tk to run event loops without the GUI, by withdrawing the mainwindow.