Why do engineers design bridges with expansion slots between the sections?

All materials expand when heated and contract when cooled.

Bridges are especially vulnerable because the entire bridge is surrounded by air, and it doesn't have the ground below to act as a heat sink.

Without the expansion slots, the bridge would run the risk of buckling when heating, and pulling apart when cooling. This could damage a bridge.

Uncontrolled or unsupported cracks, for example in the middle of a span could be very dangerous.