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.