Young Brunette Girl At Home On The White Sofa Chair

A couch, bench, or other arrangement of seats next to each other may have armrest at the sides and/or arm rests in between. The latter may be provided for comfort, but also for privacy e.g. in public transport and other public places, and to prevent lying on the bench. Arm rests reduce both desired and undesired proximity. A loveseat in particular, has no armrest in between.
There are also seats in movie theaters, and pictures of benches with and without arm rests.
Chair seats vary widely in construction and may or may not match construction of the chair's back (backrest).