 What is the purpose of the Orthan taking over a human host?
The Orthan entered the narrow strip of trees and crossed to the little-used highway on the other side. Here he patiently sat down to wait for an Earthman or an Earthwoman to pass. His task now was to bring a native, intact if possible, back to the carefully buried space cruiser where his two fellows and himself would drain the creature's mentality of all its knowledge. In this way they could learn whether a planet was suited for colonization by later swarms of Orthans. Sunlight, plenty of water and a dense atmospheric envelope made of 72-P-3 a paradise among planets. The mighty muscles of the Orthan sent him hurtling across the intervening space in two prodigious bounds, and his hands clamped across the mouth and neck of the stranger.... Then he had no time for further speech, the massive arms of the stranger had wound around him and two hamlike hands shut off his speech and his wind. "With a psychic relay we will transfer the Earthman's memories and meager store of knowledge to the brain of Thig! He can then go out and scout this world without arousing suspicion. They were trained antlike from childhood that only the growth and power of the Horde were of any moment. So it was that the bodies of the Earthman and the Orthan were strapped on two parallel tables of chill metal and the twin helmets, linked to one another by the intricacies of the psychic relay, put upon their heads. My new body must return to its barbaric household before undue attention is aroused.
 To investigate the planet without vslling attention, and determine if it's worth colonizing.
 To get the full human experience, and understand what makes the planet worthwhile.  To investigate the planet without vslling attention, and determine if it's worth colonizing.  To assimilate the human host into the Hord, and add to it their knowledge.  To examine the memories of the human host, and see what knowledge they have.