A branch line clogs. A main line fails. The difference matters, because a branch stoppage is a bad evening and a main line stoppage is sewage on a floor. The good news is that mains give warning, usually for weeks, and the warnings are specific enough to act on.
The first sign is more than one fixture misbehaving at once. A slow bathroom sink is a branch problem. A slow sink, a gurgling tub and a toilet that bubbles when the washer drains are the same problem seen from three places, and that problem is downstream of all of them.
The second is the toilet reacting to something else. If flushing the tub or running the laundry makes the lowest toilet in the house gurgle or drop its water level, air is being pulled through the trap because the main can't move water and air past a restriction.
Third is the water coming back up somewhere lower than where it went down. Water in a shower pan after running the washer is a classic. Fourth is a smell — a sewer odor around a floor drain or a cleanout usually means the line is holding standing waste instead of carrying it away. Fifth, and easiest to miss, is a stoppage you have already cleared once this year. Lines that back up on a schedule are telling you the pipe itself has a defect: roots at a joint, an offset, a belly holding water, or cast iron scaled down to a fraction of its diameter.
Any of those is worth a camera pass before the next holiday weekend, not after. Twenty minutes of footage tells you whether you have a maintenance item or a section of pipe that needs replacing, and it puts a location and a depth on it so any repair is a targeted dig instead of an expensive search.