Board Thread:Speculation House/@comment-5430952-20171101200704/@comment-26397825-20171101225748

The machine is designed to move the entire soul from a vessel into a new vessel, which is how they planned to get the Maiden Powers to Pyrrha. Aura is the soul, it's just a physical manifestation of it, and the machine was made (as Qrow said) to take the soul from one thing and cram it into something - or someone - else.

As for why Groom him; well, the closer the two are to each other, the less changes that would happen, one would assume. If they're already similar in thought processes, then putting a new soul with the same thought processes wouldn't override and warp the original, it'd just change the few differences between them.