Back in the old days, you checked in for a flight when you arrived at the airport. The airline then could see that someone was missing, and give your seat to someone else. Now, if I check in for a flight online 24 hours early, the airline doesn't know if I'm sitting at the gate waiting to get on the plane, or sitting in traffic trying to get to the airport.
My question is, if I elect to check in online before I get to the airport, and then I don't get on the plane, what happens to me? Will they let me board the next flight? Will they make me pay a change fee? Will they throw away my ticket and make me buy a new one?
If a general answer is not possible, let's say this is for a U.S. domestic flight on Delta.