“Good customer service costs less than bad customer service.”
Airlines routinely sell a few more tickets than the seats available on a given flight. Usually, this policy is fine because a couple of passengers do not show up for every trip. Occasionally, however, every passenger shows up, and the airlines have a problem.
They have to deny someone permission to board a flight even though the customer holds a confirmed flight ticket. When this occurs, airlines have to bump some passengers to a later flight.
A bumped passenger is, according to the US Department of Transportation’s website, entitled to:
cash compensation, not just a voucher, and a seat on a later flight. Bumped passengers whose travel is delayed for at least an hour are entitled to up to $1,350 in compensation, with the amount based on the length of the delay and the one-way price of the ticket.
Not long ago, United Airlines received bad publicity (justifiably) when they involuntary bumped a doctor off of one of its flights. (They also shockingly got airline police to drag the passenger off the plane physically!)
No one in the press noted that many travelers volunteer to be bumped.
I volunteered to be bumped three times and received credit toward a future flight. All three times, I have only had to wait an extra hour or two for the next available flight and I got a free flight for my time.
While I have never set out to be bumped, I have known people who try to get “bumped” on every flight. Read more about this from Johnny Jet.