This is where the U.S. airlines win hands down... In my previous days travel on American or United if I arrived at the airport early or made it to the connecting gate I was put on standby for the earlier flight. I think the official rules was a small fee $50-70 applied if you didn't have frequent flyer status but I was never once asked to pay it.

It just makes good sense to allow passangers not necessarily to reserve but to standby for an earlier flight. If the 12:00 flight is leaving with an empty seat and I'm at the airport ready to travel with a ticket for the 14:00.

That gives them both an additional 2 hours to sell a seat that would have gone empty and additional flexibility to operate their flights. If the later flight is cancelled or delayed. That's one less passanger to find a seat, re-route or accomadate in a hotel.