When things work correctly, it matters. Right now lemmy's sorting system is bugged, so just using "new" is the best way to find content.

But, when it works, the upvotes and downvotes determine how much visibility a post is given. It's basically a way for us users to tell the site what content is good, and what content is bad. If you see a thread with interesting discussion, or that links a fun video, or features a pretty art piece, upvoting will help more people find it.

If you see someone link to misinformation, or push a conspiracy theory, you can downvote to the tell the system that it is bad content, and it will show it to less people going forward.

Why you said makes me think the number of votes is wholly irrelevent.

What is interesting or helpful is entirely subjective, it's personal opinion. What is considered misinformation is entirely subjective. That makes me believe the voting count on a post means nothing for indicating the quality.

Considering how any majority of people typically react emotionally rather than have humility and respond with consistant logic, it seems personal opinion on a mass scale is an unreliable gage for quality of material.