The idea is to gauge community interest/relevance and facilitate content discovery. I feel it is becoming a bit dated method of accomplishing this and easily gamed.
You're right, there is only up/down vote systems with a user base that is in no way verified or otherwise restricted to a single vote/real person, or corporate algos.
There are plenty of different models. Do I fault the Lemmy devs for using it? No. Is it ideal for content discovery? Not really.
Why vote on posts?
What is the purpose of voting up or down on? I'm not clear what voting is suposed to achieve?...