Goldwater seemed to picked Miller simply because he was a good party man. He said that Miller "drives Johnson nuts", but it's unlikely Johnson had any idea who he was. It seems to be a Nixon/Agnew issue: no one knows who the running mate is, but that is the point because he does not detract from the main candidate. On the other hand, not many other Republicans would be willing to run with Goldwater, which is another reason he likely went with Miller.