The Friend feature, at it's core, is the same as it was before (though I think it was called the buddy and ignore lists then). It adds a plus next to their name for better recognition and keeping track of them (if you can't manually do this...), and since the 3.7 version, when vBulletin went all social networking on profile, it adds them to your profile page. That's about it. You can also disable messages from everyone but those on your friends list like Squall-Leonhart says, but it has no real special use otherwise.
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