Seat are used in many ways. Like putting it on or near tables, boats (to add extra seats), cart (to add extra seats) and many more.
A player sitting on a Wood seat
Crafting[]
Wood seat - Small Bush Stump (create item)
Stone seat - Small Stone ( create item )
Sandstone seat - Sandstone (create item)
Person sitting on mud seat
Mud seat - Pile of Mud or Small Pile of Mud (create item)