Rifts are magical gateways that link the human lands from the dead world and act as the source of all magic in both. The Order, and subsequently you as a War Mage of The Order, have sworn to protect these rifts so as to keep The Mob from invading your homelands, so failing to do so is probably a bad thing... Just saying.
The War Mage states through his dialog that the Rift itself is also a power source and has a form of inteliegence as the game progresses.
"I feel more connected to the Rifts every time I defend one, its like they're filling my head with... ah.. more... smartness?"