Airlines routinely sell more tickets on a plane than there are seats. They expect no-shows. Most of the time this causes no problems. If there is trouble, often they can find volunteers who will receive some compensation to take a later flight. Once in a rare while, airlines have to involuntarily bump someone. This explains your rights if this happens to you and why I am glad I have volunteered to be bumped a couple of times.
Unbeknownst to most consumers, the cause of most of our airline-related complaints is the hub-spoke model. Unfortunately, however, the hub-spoke model is also essential to the airline industry’s financial viability.