Georgina has it nailed above - very simple to "check in". I just set a reminder to do it the day before a trip, and it can save a LOT of time at the airport. Last trip we went with my sister and her family - they took 35 minutes to do the counter after a late arriving flight when there was only one agent working. I was to baggage claim and driving out of the rental garage about 15 minutes after our plane door opened. Simple.
And if anything, there's less pressure to add things on when you check in online.