Online check-in is advantageous for several reasons. The sooner you check in, the sooner you’re able to pick your seat, if you haven’t already.
If you’re flying on a basic economy ticket, this will be the first opportunity for you to select a seat. Seat selection is first-come, first-served, so it’s important to check in as soon as it’s allowed.
You’re also able to pay for an upgrade at this point—again, this is based on availability, so it’s in your best interest to check in sooner rather than later if you’re hoping to pay for an upgrade to a premium class.
If a flight is oversold, those who checked in last might have a higher risk of being bumped from the flight (though this varies by airline and fare, and involuntary bumps are rare; the airline will start by offering vouchers to volunteers.)
If you're not checking luggage, you can skip the check-in desk at the airport and head right to security.
Additionally, it’s important to check in online—as close to the 24-hour mark as possible—when flying on Southwest, as Southwest has open seating and boarding order is determined by check-in order (though there is an option to pay more for automatic early check-in).
At check-in, Southwest passengers are assigned a letter, starting with A, and a number 1-60. Group A boards first (with A1 the first passenger within the group), followed by Group B, etc. The longer you wait to check in for your flight, the farther back you’ll be in this boarding line. If you’re one of the last people to board, you’re choice of seats will be very limited.