I understand the idea of voting for a great comment, but why does the comment's score change colors at five-ish up votes? Comment votes don't seem to give reputation, or anything else, for being great.
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All the votes do is help to visually identify good or insightful comments. You are correct, the color of the vote count does change at (IIRC) around 5 upvotes, and then again around 10. The specific color is tied to the site you're on; for example, the color becomes more orange on Stack Overflow.
Also, when the page initially loads, there will be at most 5 comments below each post. The rest of the comments can be expanded out, but the initial comments are the five highest-scoring.
Other than that, the score serves no purpose. However, I will say that even though I don't get reputation from a comment vote, I personally like to see that someone liked my comment and thought it was good or insightful.
For more information, see http://blog.stackoverflow.com/2009/04/comments-top-n-shown/
I still think there should be a badge, even a bronze one, for getting 10 upvotes on a comment.
Expanding on Kyle, the primary purpose of them was to get the best comments loaded on the page for all requests (instead of via AJAX) - so they can be indexed by search engines/visible to all users.