New user registration on your site – what is this & how can I utilize it?

I got 2 emails in the last week now about “new user registration on your site”.

Both users have strange emails.

New user registration on your site NewsBall Inc.:

@poczta.pl

and

@o2.pl

How are people registering for my website newsball.com? I removed that whole RSS FEED link stuff from my right column. People can post comments giving their name and email, but there is no need to register. So how are they registering? And what purpose is there in having readers register to your blog? Is there an advantage? Is anyone’s wordpress letting readers register, and if so, how do you utilize that? My other site dollarscholar.com has 72 registered users and I have no clue who they are or what to do with that list or how they registered.