Wordpress Answers requires users to sign-in before answering a question or leaving a comment. But voting on questions is anonymous. Why is that?
First and foremost, it is common in the StackExchange Network.
Second, it may have a negative effect if users can see who rated them down, as some people may have revenge votes.
Jeff Atwood, co-founder of Stack Exchange, wrote1:
The idea of a world where nobody can be downvoted strikes me as more than a little utopian. Is it realistic for users to expect to post in an environment where there are no penalties at all, no way for their peers to express disapproval or disagreement with their post? When you can't leave a quiet, anonymous downvote, you're more likely to post a snarky reply to express your displeasure. That’s why disallowing downvotes is actively harmful to community.
In other words, by giving users an anonymous downvote, the site prevents more noisy forms of disagreement. The same (more or less) holds true for upvotes. We don't have a lot of "me too" comments because the upvote sends the same message (only better!).
- Emphasis mine. The original post was mostly concerned with downvotes, but the principle applies to all voting.