You have probably never even considered writing a function that calls another, not yet defined function, and just moving on because your trail of thought is headed elsewhere at that very moment. Fifteen minutes later you are testing your new function and the debugger breaks on the call to your still undefined function. "Oh right", you think, "I need to transmogrify the foo here". So you define the transmogrify-foo function and continue debugging right there.

This isn't contrived or exotic in any way. It's just second nature when writing lisp in an interactive fashion.