Makes a lot of sense. A lot of people would otherwise just take their original flight - which DOES NOT make sense.

For example, Joe shows up at airport 3 hours early. Next AAA/BBB flight is in 45 mins with 20 empty Y seats. He wants on it. His original flight is just about sold out. Offloading him opens up 1 Y seat. Mark shows up at airport an hour later and there's a full Y fare that UA sells him and Joe is already out of town. It always makes sense to get people out of the originating airport ASAP if there are seats. It also makes sense not to penalize people if there was some traffic or security nightmare by offering a same-day standby seat.