I had a flight that was delayed by 2-3 hours about a year ago. I went to the airport and saw that the flight was delayed so I left to come back an hour later without checking in. I tried to check in after the scheduled flight time but before the delayed flight time but I was told that checkin was already closed. The checkin agent had to call the gate agent to ask if there were seats available still for me to occupy. In the end I flew on that flight but I could easily have been locked out of the flight if it was full.