I once asked someone "how is this function used?" He put a breakpoint in it, ran the code, triggered it, and showed me a stack trace. "Here's how it's called." That was a nice specific example, but I wanted an understanding of intent or architecture or something he clearly didn't have. I almost never use a debugger, but my approach was developed in a time of 8-bit machines that didn't have debuggers. That said, I have seen bugs that could probably not be found without a debugger - including compiler errors.