Registration Spam

Hello All
I have looked in the documentation and searched the forums. I found nothing to help me understand why so many people register, what is the point? I’ve had over 500 registrations by obviously fake people. None of these registered users ever try to post anything, or is that kaptcha preventing them from posting if so it doesn’t show in the log! There has to be some devious plan these people are trying to carry out. So far I (crossing my fingers) have not had any problems due to this, just the constant removing of junk users. I recently installed plugin registration plus hoping this will help. If anyone knows What These People are doing this for and why most of them live in USSR (or what ever they call it these days) or Gmailville. Please give me a clue.
faz67