Question

Bridges are mounted on rollers. Why?

Answer 1

There are two reasons: First, as a bridge deflects under load, the ends rotate a small amount and pull inward. The rocker/roller allows for this movement. To see this happen, take a thin stick, support it at the ends, and push down on the middle; the ends will rotate and pull together a little bit.
Second, a bridge will expand and contract due to temperature changes.The rocker/roller lets the bridge expand and contract.