The great Sisters had moons once, and it can be said that they still do. ==>More
Today the Tether serves as a conduit of magical energy, with the sole purpose of interplanetary travel. The power needed is greater than all but the most powerful mages to conjure, which is why the process is mostly automated nowadays. Conduit sits at the best possible spot to jump where the planets are the closest and the Tether is the most focused.