Originally Posted by
redtop43
The only real reason you need to check in is to get a boarding pass, if you don't want to stop at the kiosk at the airport.
It doesn't foreclose anything else.
It may not "foreclose" anything else, but if you're not checked in 45 minutes prior to boarding (or 60 minutes for international flights), you may be foreclosed from getting on the flight.
Generally speaking it's a good idea to check in online if you think there's any chance you might run late (but still make it to the gate in time to board).