I was thinking that HN shows only the writer what the 'value' of the post is (number of upvotes). In Reddit, most subs seem to show that number to the whole world.

So if this comment had 100 upvotes, and another one 20, you're the winner, everyone can see it. I speculate it at the minimum encourages subconscious behavior of trying to game votes upwards, and also biases people that it's a good comment. Because your brain will process "100" before reading a paragraph of words.