...we do commercial construction, so our plumbers very often have to use their camera to locate the main drain in order to intercept it below the floor slab to add a branch line for new fixtures.

We recently had to get a main sewer line surveyed because of a backup that affected two different buildings that are served by the same lateral line. This part of town has combination storm + sewer lines, and back in August there was a torrential rain of ~3" in an hour that overwhelmed the combination system. There were no immediate issues then, but a month or two later, suddenly the sewer system backed up in one of the two buildings. While one of our guys was on site investigating, he opened a clean out near the street and saw leaves come out with the water that was forced out. Then the clog apparently cleared itself and the line cleared almost as quickly as it had backed up. We had the line inspected by camera all the way out to the manhole in the street and could see that the last ~30' consisted of old terra cotta pipe. The only thing we could figure is that the rainstorm had forced leaves/debris from the storm sewer inlets back up into the lateral, where some of it got hung on the terra cotta pipe joints. I'm guessing that over time, enough waste got piled up behind that debris that it eventually formed a plug and totally blocked the line.

Needless to say, the drain camera is a handy tool.