The term "seat backs" refers to the rear portion of a seat that can be reclined or adjusted. It is typically located on an airplane, bus, train, or in a movie theatre. The purpose of having seat backs is to provide additional comfort and support for passengers while they are seated. They allow people to lean back and rest their head and neck against the backrest, which can help reduce fatigue during long journeys. Additionally, some seatbacks have built-in features such as tray tables or entertainment systems that make them more versatile and functional for passengers.