I've noticed for my local races it's about 75% online and 25% day of.
IMHO it helps the event organizers to register earlier so they have funds-in-pocket to pay for things needed at the race, day-of, like EMS, Police, Barricades, Personnel/Staff, etc.
Some events have a cap on how many participants. For hot events, it helps to register early to secure your spot.
Otherwise, it doesn't matter really.