The #1 reason for online check in for me (as you say, when not checking bags) is that if unexpected traffic etc makes you miss the airport check in cutoff time, you can still get on the flight. But for this, it doesn't matter whether you check in right at T-24, or wait until an hour before while sitting in traffic en route to the airport.

One reason to check in ASAP that hasn't been mentioned:
Some airlines (e.g. AA) use check in time as the tiebreak for standby lists. Even if you aren't planning to standby, flight cancellations or missed connections may leave you on a standby list for a full alternate flight, and an earlier original check in may be the difference between getting that last seat or not.