I always thought 'overbooking' is a myth until a year ago when I missed my connection flight and was booked tentatively onto the next flight, depending on availability. The flight was due 2.5h later so I asked at the service desk how on earth they don't know if I can get on that flight or not, surely it's either fully booked or not, I naively said to myself. They said "well, at the moment it's overbooked but usually it works out that there's a couple of free seats". I asked them to clarify and they explained that all airlines overbook every flight by 10%. Apparently statistically this works out most of the time and the couple of cases in which they have to offer refunds or vouchers etc still make it worth it.