From my perspective there isn't very much difference between using the debugger and sprinkling print statements through your code. In either case you're running your program in a way which gives you information about its internal state as it runs. The biggest difference is the level of interactivity: the debugger offers immediate access during a run but print statements require a new run to take affect.

I've used the debugger a few times, most often when I'm working on a program that takes a long time to get into a bad state. In this case the extra time needed to add a new print statement and get the result after a new run is too high.

Another good use is to figure out what your program is doing when it's locked up and you can't guess a good spot to put a print. The debugger can answer that question very quickly indeed!

-sam