The Seat object creates a brick that when touched by a player will anchor their torso to the brick and move their arms and legs forward, in a sitting position just above the brick. This allows for the creation of vehicles that players can sit in and not be flung about the cabin/deck of the vehicle.
The Seat object is very useful for letting people sit. Be it a park bench, a roller coaster, a seat on a bus or a plane, anything where you want someone to sit down and not move. When a player sits on the seat a Weld constraint is created, so they are 'attached' to the seat and can't move until that weld is broken. It can be removed by hitting the spacebar, when the player jumps to exit the seat.