Bridges grow slightly longer as their materials warm and contract when they cool. Over a long span, even a modest change per meter becomes movement that supports and pavement connections must accommodate.
A small change across a long span
Deck expansion joints provide a gap that can open and close while vehicles cross it. Bearings may allow the superstructure to move relative to its supports. Engineers also account for shrinkage, creep and other effects depending on the bridge. If movement is restrained, stress can build in the structure or its connections.
Joints are one design choice
Joints are not inevitable in every design. Some integral or jointless bridges transfer thermal movement into flexible abutments and foundations instead. The choice trades maintenance and detailing against how the whole structure responds. A visible joint is therefore part of a broader movement strategy, not merely a crack left in the road.
Related: why concrete needs steel reinforcement.
Source
US Federal Highway Administration: Bridge Deck Expansion Joints.