The benefits of checking in for your Southwest flight online include getting a good boarding position, avoiding the check-in line at the airport and going straight to the security checkpoint. If you purchase EarlyBird Check-In, the system checks you in automatically 36 hours before your flight.
As of 2015, Southwest Airlines has an open-seating policy that allows travelers to choose their own seats, in the order that they checked in for the flight. When you check in for any Southwest flight, you are assigned an A, B or C boarding group and a boarding position, 1 through 60, which is the order you board the plane with your group. You can check in for your flight online up to 24 hours in advance if you purchased a standard ticket. Checking in as early as possible gives you the best chance to get in the A or B boarding group, giving you a wider selection of seats to choose from.
Checking in online is also convenient because it means you can skip the ines at the check-in counter at the airport. Instead, you can go straight to the security checkpoint. If you haven't printed your boarding pass at home, go to one of the check-in computer kiosks to print your boarding pass before heading through security. Even if you have luggage to check, there is typically a separate, shorter line for checking in bags if you have already checked in online.