Ok, call me a pessimist (because I am, so does the OC:Downvote pony), but the original purpose of it is… having this feature is encouraging artists to draw quality arts, and use the score combining with comments for constructive criticism, and bad arts are less likely to be continued to be uploaded, keeping the MLP franchise name in high quality, and not looking too childish as what society stereotyped.
But so, nowadays, the there exists Manebooru, quality arts scores as always, still, but bad arts would remain low score (much like DeviantArt and Pixiv, only favs, no downvotes), so…
I don’t know if Downvotes (and dislikes) are still important these days, nowadays, you dislike something, you can just ignore it, or report it if it really is too offensive. Reddit doesn’t even show posts those are negative scores, but comments can still have negative score in order to detect the defect of the subreddit.