They do it to harvest the juicy bits from the dead to manufacture a drug that allows them to live as gods. DMT. Ambrosia. The "spice". This is what "Dune" is really about. Harvest a drug from the worms (us), but don't wake the worms by being too obvious about it, and above all regardless of the conflict between royal families, there is really only one prime directive: the spice must flow.