A few modern campaign questions

[Sergio65]

Here's how I see it. A bridging unit's primary function is to build and maintain bridges. Lose the bridge and the unit serves no useful purpose other than as a poorly equipped rear area garrison.

So why then would a bridging unit make the decision to lose their own bridge?

The answer, imo, is immediate threat, as in 'enemy tank battalion rolls up to the opposite side of the river' kind of immediate. That kind of urgency would not allow for much time to disable the bridge.