The Forerunners built the Ark to be completely self-sustaining, and much of its power lies in the vast subterranean web of power structures beneath its surface. These external power nodes were designed to provide auxiliary power during repairs or emergencies, giving those in control of them a steady flow of power resources.
On this remote installation, light years from any support, both the UNSC and the Banished have realized the strategic importance of Forerunner power nodes. Eschewing the Covenant's reverence for Forerunner artifacts, Atriox has no compunction of taking control of them for his own ends, and many a battle involves a desperate scramble to capture power nodes before the enemy can.