Online check-in is available 24 hours prior to flight until 30-90 minutes before departure time.
Print boarding pass ahead of time saving you time at the airport. Some airlines also offer the option of downloading your boarding pass to a PDA such as a Blackberry.
Ability to change/confirm your seat assignment when available, thus a better chance at getting a seat number before you are even at the airport if you didn’t have one already pre-reserved.
You may check in for return flight, as well, so long as trip is within 24 hours of initial check-in.