This tactic is one that is used just to simply deny any
enemy forces a clear path of travel. This can be done by forcing individual
unit fire down a corridor, grenade spamming open ground, using a Machina
Missile Tank to arbitrarily target ground, etc.
Makes the enemy either think twice about proceeding through a section of territory or suffer damage if they are moving through it. Works well when used with Short Bounce Grenading with a Grenadier running through a building or grenade spamming down a hallway with advancing enemies.
Denial of area lets you buy time. Always gang up on your enemy, never take them head-on to avoid losses of health or units. If you are outnumbered or can't reach your enemy, lay down a denial of area attack and run the hell away.
Denial of area also allows you to not worry about certain areas - if a horde of Spawn are rushing from all directions, and two of those directions have at least two Grenadiers bombing them, keep your opportunity fire focused elsewhere for most of your force. If a tight hallway has two or more Marines shooting Suppressive Fire down it for the duration of the turn, anything coming through is going to get hit at least a couple times.