[Their] M.O. is to gain tech, destroy the civilization itself, move to a new civilization. rinse and repeat until all relatively advanced civilizations are gone.
At that point they have ALL superior tech and watch and gain global control, as civilizations advance slowly from a hunting gathering or agrarian to more advanced stages.
If a civilization comes up with new things or advances too rapidly (with or without [Their]) influence they take everything worthwhile and level it and start over?
Each cycle gives them Superior god-like (to the masses) power, and inevitably a point comes where they control the worlds people fully so they can build the world in the way they want, provided they fully control people and reproduction primarily, all life and death?