One thing about the old system was that the airlines know you were (or were not) in the airport, so they could fill empty seats. These days there must be some no-shows who have OLCI'd. In the US, there is of course gate standby, so the seats can be filled (though of course there is often an unholy scrum at the gate) so unoccupied seats can be filled.

Remember the days of telephone check-in?