Personally my theory is that's it is farming races for light. Creating artificial golden ages and then draining all the energy they create (there have also been hints at a Fallen utopia that was eventually invaded by the vex). I think the traveler uses the energy (light) as fuel, and cultivates then leaves. The darkness that follows is either a conglomeration of pissed off marks, enemies, or some other relation entirely. The light we use is just a side effect of it being stalled out by Rasputin. Then again thats just my geeked out theory.