By checking in early, you're less likely to be bumped - flights oversell regularly, on the assumption a certain percentage won't make it. As a result, if you really want to make sure you get on the flight, you check in ASAP. If you've got status, it gives you more time in the lounges, to eat, drink and relax.

Generally, it is better to check in as early as possible, typically right when the 24-hour window opens. Checking in early allows you the best selection of seats (if you haven't pre-paid), increases your chances of being eligible for "Saver" upgrades, and—most importantly—reduces your risk of being "bumped" if the flight is overbooked. Airlines often use "time of check-in" as one of the criteria for determining who stays on the plane in an overselling situation. However, there is a minor "hack" for checking in later: if the flight is full, the last people to check in are sometimes assigned "premium" seats or exit rows for free because the standard seats are all taken. This is a risky strategy, though, as you are equally likely to end up in a middle seat at the back of the plane or on the standby list. For peace of mind and better logistics, early check-in via the airline's app is almost always the superior choice.