Unwanted and unrequested registrations keep being made on my website. What is the motivation for these people to do this? Are they done by computers? The addresses they fill in are from around the globe. I don't know or care to know these people. What's really going on here?

There's really no telling what is happening.

The best choices are:
1. Remove registrations -- this is a setting in site settings
2. Make registration "verified"