Airlines overbook because a predictable percentage of passengers don't show up for every flight — cancellations, missed connections, and no-shows are baked into historical data for each route. Overbooking lets airlines fill seats that would otherwise fly empty, which keeps average fares lower for everyone.
When a flight ends up oversold and every ticketed passenger shows up, the airline first asks for volunteers to give up their seat in exchange for compensation — usually a travel voucher, sometimes cash, plus rebooking on a later flight. If not enough people volunteer, the airline can involuntarily bump passengers, usually based on check-in time, fare class, and loyalty status.
Volunteering can actually be a good deal if your schedule is flexible — vouchers for a bumped seat are often worth several hundred dollars, sometimes more during high-demand periods, plus meal or hotel vouchers if the delay is long. Involuntary bumping comes with legally required compensation in many countries, calculated based on how much later you arrive at your final destination.
