The last couple of times I tried to check in on-line, United decided it couldn't print me a boarding pass anyway because it wanted someone to have sight of my travel documents. So I ended up in the queue anyway. I always prefer to get the boarding pass at the airport anyway, less chance of losing it. I know with some airlines it's possible to do the whole check-in on-line and then get the passes from a machine at the airport, then just drop the bags.
I think part of it is that the whole list of questions about "did you pack it yourself" etc are part of the on-line process and so don't need to be asked at the desk, saving some time.