Now, other readers have information that they would not have had about your post. They know this is a post that sigmajin likes. Which they probably don't care if they don't know me. But if they do it might mean something to them about whether its worth reading.
In the aggregate it could mean a lot. Wisdom of the crowd and all. Maybe they don't know or care who sigmajin is or about your stamp of approval but if something has many votes then, in theory, later readers might find that very useful information.
In the same way downvotes could be useful information too, in a different design that didn't discourage them. Someone seeing a post with few upvotes now has no idea whether it wasn't liked or wasn't considered. A downvote conveys that it was considered and judged to be poor.