The armrest is there to divide the bench
A bar set across the middle of a public bench is usually intentional. It separates the seat into individual spaces and makes it difficult to lie down across the bench. Designers and public authorities may use that arrangement to keep benches available for seated use, especially in busy areas where space is limited.
The same feature is often described as hostile architecture: design that discourages certain behaviors by making them physically harder. A center armrest can deter someone from sleeping on the bench, though it does not address why they need a place to sleep. The term “random” can be misleading—the placement is generally meant to interrupt the long, flat surface.
A barrier can also help someone sit or stand
Armrests have a practical use for seated visitors, too. Someone with limited strength or balance can push down on an armrest when lowering onto a seat or rising from it. Dividing a bench into smaller sitting spaces may also give people a clearer sense of personal space.
Those purposes can conflict. A design that helps one person stand may make the bench less comfortable for a group, and a fixed divider can leave little room for someone who needs more space. There is no single reason behind every bench: the choice reflects local priorities about access, comfort, maintenance, and how public space is used. The middle bar turns an open bench into a set of separate seats—and makes lying across it much harder.
