It isn’t for no reason other than that. Granted, that is a reason. However, without a clear ratio of upvotes to downvotes, there’s no differentiating between good vs bad. YouTube removed their dislike button, but you can still see how many views a video has. If a video has 1m views but like 3k likes, you can see a ratio there. On here, there’s no ‘views’ per comment. So a use case would be that, say I posted something objectively false that supported a narrative. If others come along and support the same narrative and upvoted it, that post would look relatively credible. Downvotes allows for democratic running of social media. Which is what Reddit once was, and what lemmy is trying to be.
I know it’s a shit thing to say, but if your posts always get downvoted to oblivion, it’s probably moreso you that’s got a skewed outlook vs everyone else. (Not you as in you but you as in the general person)