With the power of defending the systems they are installed in, and by changing their offensive abilities. In my experience, you can just roll on everyone with arks really quickly without your ennemies being able to keep up with your firepower. I think it's a shame that they doesn't count as an orbital defense in the systems they occupy while still being able to invade really easilly other worlds because of their major support modules (just give it 2 manpower upgrades and you can stomp your way through the game).