To put it very simply.
Covering fire is used to enhance movement in combat situations when both sides are already engaged in a firefight and therefore freedom of movement has been restricted as the troop's own position has been spotted and is taking fire. So when they have to move, they have to be given a fighting chance and to do so, you respond by giving them covering fire. Basically you trade accuracy for sheer volume as you open up at an enemy position with your maximum available firepower and force the enemy to duck for cover and shoot from behind cover, as they can't aim from behind cover, they stand a lesser chance of hitting moving troops. However, its not a guarantee of safety of security by any measure, a platoon of mine lost five men in a 30 meter stretch as we tried to take a militant complex in Bajaur, despite the fact that we had covering fire from a nearby tank mounted 12.7.
