Various things on the battlefield like Iron/Oil nodes, flags, crates, and even some weapon systems can be captured and used. Time is needed to capture said objectives and that can leave your units vulnerable to gunfire.
When capturing an object that was controlled by the enemy, there is also the added problem of being required to decap the enemy's flag on things like resource nodes and flags beforehand.
You can stop an enemy from capturing or reviving a hero by dealing damage to the squad, which is trying it. Still will take away from progress and can delay or stop the capture.