 What is the purpose of the Orthan taking over a human host?
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. 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. Men had no mates on Ortha, sex had been overthrown with all the other primitive impulses of barbarism; so he was incapable of understanding the emotions that swept through his acquired memory.
 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.