Great observation! That slight curve on a street bench is often a deliberate design choice, and it can serve both drainage and aesthetic (plus ergonomic) purposes—though drainage is usually the primary reason.
Most outdoor benches have a very gentle convex curve (arching upward in the middle) so that rainwater runs off the sides instead of pooling on the seat. Without that curve, water would sit on the flat surface, making the bench wet for hours after a shower. The curve also helps debris like leaves and dust slide off. So in many cases, yes, it’s mainly for drainage.
That said, some benches use a concave curve (curving inward) – think of a park bench that hugs your back or bottom. That’s more about ergonomics and comfort, making the seat feel more natural and supportive. Architects and urban designers also incorporate curves to make benches visually softer, breaking up the harsh lines of city concrete. So in certain stylish plazas or modern parks, a curve might be purely aesthetic.
Bottom line: if the bench is slightly domed (highest at the center), it’s almost certainly for drainage. If it’s dish‑shaped (lowest at the center), it’s designed for comfort. And sometimes designers combine both – a subtle curve that drains water but also looks elegant. Next time you sit down, run your hand across the surface and you’ll know which kind it is!