[K]

I'll do it when my opponent has broken a NAP, bargained in bad faith, or been spamming me with messages. In short, jerk behavior as payment for jerk behavior.

Otherwise, I'll do it to prevent a key asset from being in enemy control. For example, a site producing Wizards will have its lab burned and provinces overtaxed so that an enemy taking it won't get much use out of it before I can retake it.