Downvoting pushes peoples comments down and greys them out, effectively silencing them. It creates echo chambers.

I reserve my downvotes for when arguments are made in bad faith, rely on logical fallacies, or present know-false information as an argument.

If someone presents an argument on something I disagree with, but it's made in good faith and is well-structured, it deserves an upvote, even if I still disagree afterwards.