It came to me in the flash of light I always see as the grays are putting my paralyzed body back in my bed after the abduction. I know what they want.
They are universal search bots.
Really. They roam the real universe picking up living objects, extracting metainformation from them, and posting it back to the 5D Universal Search Engine being run in the dark region beyond Saggitarius. As new alien races sign up to the free services, they can fly or instantly be teleported to the last known location of the object in 5D.
It makes sense. Shhh! Tell no one. I filed for the patent. I'm selling it to Rupert Murdoch.