overbooking helps airlines limit the number of empty seats on their flights.
However, if a flight is overbooked, some passengers with confirmed reservations have to get off the plane.
Getting bumped isn't always a bad thing, however. There is a growing number of passengers who feel lucky if their flight is overbooked.
Why? Because airlines have to provide bumped passengers with cash, free flights, hotels, and/or meals to compensate them for their inconvernience.
In fact, airlines usually ask for volunteers to get off an overbooked flight in exchange for those perks, and many passengers say "Sure!" and happily deplane. Some people even make a habit of choosing flights that are likely to be overbooked, just so they can volunteer!