Honestly, I used to think using a camera for every sweep was a waste of time. I figured if the brush went through clean, that was good enough. Last month, on a job in an old brick house, I almost skipped the camera to finish early. But I used it anyway. I saw a huge crack in the clay liner that wasn't visible from below. That scared me. If I had missed it, the chimney could have failed. Now, I always run the camera first. It adds time but prevents problems.
Using an old brush on drywall once, I saw a crack form right after smoothing it out. That feeling is the worst. You think you're done, and then it's all ruined. Probably means the surface wasn't prepped right. Now you have to fix it before moving on.