It tells the airline that you are actually intending to fly as booked.
A lot of airlines maximise revenue by overbooking, and checking in helps them deal with potential no shows and/ or too many passengers showing up for a flight. With online check in, they can be pretty sure that you will show up so they can worry about getting everyone on board.
Common sense really - and there are a lot of folk who book a flight and simply don't show up for it for whatever reason