The gap that prompts the lab report is between what the teacher expects you to learn from the lab and what you have learned. The report is a way of seeing how closely those two things match.
Lab reports are nothing but busy work.
All they are good for is wasting your time. That's the way most students see lab reports. But there's more to it than that. Look at it from the professor's perspective. The goal is to use labs to help teach the course material by giving you hands-on work with the concepts laid down in class.