It really depends on the program architecture and what issue you're debugging.

For most single-threaded or simple multithreaded programs, it's easier just to throw a print statement where you need it and analyze that. Even with a large data set, a simple grep will probably get you what you need.

When you're debugging a complex, multithreaded program, on the other hand, an external debugger is much more valuable, because you can break at the exact point where things go wrong, and examine the entire program's behavior at that point. Debugging a race condition with prints can be pretty difficult, especially when the printing changes the timings of the threads.