If your vote is simply reflecting the information content already available from the other votes, what have you added?

In that case I don't have added any information but I also didn't put any wrong information into the system. In most cases voting will be a combination of adding new information and repeating back information that already available.

It's not to pat yourself on the back for upvoting useful posts.

While not being the main point of voting, feeling good about upvoting useful post is desirable. Having a sense of community is good. Cooperation among people on LW is good. We don't want to foster an environment where everyone feels like his on his own but a environment where people feel like they are cooperating with each other.

you shouldn't be asking "Do I think this post is useful?", but "Is this post, given the current vote total more likely to be useful than other posts with the same post total?"

I disagree. If a lot of people find a post useful because it helped them, I think that's a valuable signal for the person who wrote the post. Answering an easy question when voting leads to more voting than asking a more complicated question.