Ukraine



To answer that question, there are two reasons.

If the bomb explodes and punches through the deck the blast wave will hit the ground or water underneath and reflect back up. At the end the gap between the bridge and ground or water is closer and more of that reflected blast wave will do additional damage to the bridge. And if the bomb near misses the bridge off to one side, the blast wave lofting upward might still bring down the bridge.

The second reason is it is easier to make a repair to a bridge that is broken near the end. Just as soon as the enemy has been beaten in that area, your own side will likely need to fix that bridge to continue to pursue them.

Usually that is done by placing a short temporary bridge over the broken part.