WordPress blogs (and some other websites) require email addresses. The email addresses are only used for sending notification of replies, isn't it? (or a kind of CAPTCHA). Some people are reluctant to type their real email or just give up leaving a comment. The owners may miss some opinions from those people then.
Sample page:
http://wpthemetutorial.com/2013/05/02/working-with-vagrant-and-wordpress-basics/
What email address in comment box?<p><a href="https://news.ycombinator.com/user?id=sirtel" rel="nofollow">https://news.ycombinator.com/user?id=sirtel</a> has a field for email address, but that is a profile page, not a comment box. Did you mean to write "profile page"?