As the only damage dealing addon, The Defense Turret's primary function is to attack enemy units that tresspass on your territory to prevent them from damaging your HQ. Every few seconds, the Defense Turret will launch a mortar strike, very similar to the Mortar Cannon from Republic Defender, however it can only deal damage to one enemy at a time and deals a considerable amount of damage, even to vehicles. The Defense Turret also has a long reach, being able to launch mortars at enemies that are at the very far end of your territory. When upgraded to Rank 2, the Defense Turret gains more attack power and the ability to damage multiple targets per shot. The attack power is increased further when upgraded to Rank 3.
Defense Turrets should only be used when enemy units encroach on your territory in threatening numbers. If there are no enemies within your territory, using a team benefitting addon such as the Armory or Supply Center can be better to use. As said before, Defense Turrets are very powerful, and can most of the times take out small groups of units. If necessary, Defense turrets can be sold for 50 energy.