Subscribers

What is the purpose of having users subscribe to your site? I’ve been using WP for a couple of years and have people who subscribe to my feeds, but I do not make people register to leave comments and such.

Why should they subscribe? Also, I’ve always wondered (and have asked this question before but with no answer), how do you prevent parts of your site from showing up unless a user is registered and logged in to see it?

Please help if someone knows. Thanks!