Well, I find the TDC marks real helpful when I check or adjust the valves. ;D
The timing marks are pretty much for just checking it. One mark is either for static timing or no advance at idle (can't remember) and one is at full advance. If the flywheel key ever sheers or the timing gets out of whack for some other reason it will show up cheking it with a timing light. I think some adjustment is possible by moving the timing pickups, but it ain't a casual job.