Nobody knows exactly why the Ultrethqwe tower was constructed. The Forerunners never established a significant colony on Ketesh, only using it as a stop-off point for their resupply facility orbiting a gas giant further out in the system. All indications are that the tower was part of a larger complex of structures, but suffered significant damage from an asteroid impact in approximtely 14,000 B.C.E., leaving the tower as the only remaining structure.