I think they are important as the YouTube dislike button. It helps me to double check what I’m reading in case I get fooled by something that sounds plausible enough for me. Apart from negative reactions, I don’t usually post because sometimes what I wanna say it’s not valuable enough or it doesn’t provide a real informative value to the thread, but at the same time I want to express my opinion so there’s where reactions comes in.
If someone already posted the reason I would just thumbs-up it or use another reaction so I don’t repost what the other user already said or say something spammish like “I agree with you.” and nothing else.