Eh...confirms to the airline that you're taking the flight, in case you're worried about getting to the airport late. Also gives you the opportunity to scope out seat availability again if you aren't happy with your current seats. Other than that, it's just about saving time. There should be a bag drop lane at the airport, and the lines for that are sometimes shorter than for the general check in lane. I often check in online but then print a boarding pass at the airport.