Little help over here - subscribers & registered users

What is cooking?

I am hoping some of you might be able to help me out and answer some questions I have about using wordpress, registered users, and then subscribers.

Background:

This is like my 5th site - 2nd "real" one. It has only been up for a little over a month. As people on here seem to preach that the money is in the list, I wanted to be on top of building one from the start with this site.

So, I've got mailchimp which seems to handle the opt-ins just fine.

I never paid attention to the users tab in wordpress before this site. I know that I have it set up that in order to leave a comment you must be a registered user.

1) But should I be doing something else with that info if someone becomes a registered user?

2) I see in wordpress where I can grant different access levels to people, is that why they are signing up? So they can post or comment? Like are they waiting on me to give them an access level so they can then do something?

3) Or are they registering in hopes that in the future they can get auto approved comments going for themselves?

4) These registered users - should I just be kind of adding them to my subscriber list or is that underhanded?

Thanks! I'll be checking all night for good answers.