Essentially, because it allows anonymous support for posts that get banned without that user saying as much in their own words, or having to put the effort into their own argument. It also boosts those who can make a horrific argument with elegance or guile, by allowing others to then echo and hide behind it. Much like 'will you buy it?' polls about games with problematic elements, it has the side effect of reinforcing majority views in topics around representation etc which isn't what this community is about. If people want to back another post, and there are also tons of great posts every day, they can do so with their own post rather then making a discussion forum a popularity contest.