Saves time at the airport is the major benefit, particularly if you aren't checking a bag. You never know when there will be an unexpected line or something happening that will cause you to be late. Checking in early might help in a situation where the flight is oversold -- as FlyingUnderTheRadar says, if they're going to bump someone from the flight, check-in time is a factor (and you can put your name on the "willing to volunteer list" -- although I don't think there's a benefit to doing this earlier vs. at the airport). Also, you can re-assess your seats earlier -- blocked seats may be available to select via online check-in, but still not available via just viewing your reservation (plus there is always shifting of seats in the 24 hours before a flight as people get upgraded or check-in and change their seat). For elite passengers, check-in time can be a tie-breaker for whether they get an upgrade.