Everyone was getting seats allocated at checkin, which was at a checkin desk at the airport. If you were lucky, you'd get asked if you wanted a window or an aisle.

Then you could check-in by telephone. Then you could check-in online. Then airlines started exposing the ability to select seats prior to check-in. Finally, someone realized this could be a revenue stream and started charging for it.

At this point, British Airways will make you pay to reserve a specific seat before check-in even if you're flying in intercontinental business class unless you have an expensive flexible fare or are a frequent flyer.