I believe it simply pins it to the top of the list of your stories, questions, etc. Pinned items are the first things people will see when they look at your profile.
It's just highlighting the posts a person wishes others to see first when viewing their profile, because it's important to them in some way. That is why I've "pinned" posts, anyway.
It does not guarantee anything.
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It just puts it at the top of your profile it's only seen more by people looking at your profile.. because it's right on top of you posts instead of buried somewhere (for people who make a lot of posts)