It is possible that the last minute dummy booking is in a much higher fare bucket than the existing reservation for which online check-in is open. The seating algorithm (similar to BA's "theoretical seating") would then offer a wider selecion of possibly more desirable seats to the higher fare bucket booking than the lower one.

If a passenger does not have a fare that includes seat selection and/or medium-high oneworld status, paying may be the only way to secure the more preferable seats.