Upvoted because it's generally true.

On anything controversial, the voting system is borked, just like any voting irl. Gather enough people in one place/topic and you can make the most insane thing seem true.

Pictures of kitties and boobies though? You should be able to gauge what's good and what's crap.

Yep. That's why you sometimes see people downvoted into oblivion, simply for stating something which is true, within a community that is deluded about that given thing. Whether the votes accurately represent the value of the content, depends entirely on who sees it.

But at the same time, saying it is truly pointless, would mean you also consider the very concept of democracy, pointless. Yes, there will be a percentage of people who are unable to form a level opinion, and how many such users there are can vary wildly depending on who sees a given post/comment in the first place.

But results speak for themselves. Reddit's voting system does work. Especially because when you go to a specific subreddit, its about a specific subject. Meaning the users who are there, likely align in what they are interested in, meaning the voting is now a much more accurate representation of what the subscribers of a given sub want to see. Your subjective opinion is likely to match that of the users looking at the same subreddit. And this continues working even as you subscribe to multiple subs. Each post only gets shown to users who subbed (unless on r/all), even though each user has a mixed feed of the stuff they subbed to.

What is interesting or helpful is entirely subjective

Nothing is entirely subjective, at least not in the sense that you mean.

There are different degrees of shared opinion ("inter-subjectivity") among people, depending on the group. One of the advantages of the "communities" (or "subreddit" / "magazines") model is that you can find people with whom you share opinions, and if that community doesn't already exist, you can create it.

By joining a community that shares your interests, and customizing your feed to show those communities, content that gets upvoted will tend to reflect your interests, and upvotes will be a signal of quality.

People have limited time. By having an algorithm that can sort by likes / dislikes, everyone saves time by delegating some of the time-consuming task of discovering relevant content to the algorithm.

The people downvoting you are proving your point a bit... Come on people, don't downvote something just because you don't agree. You can just not upvote it if you really want, but it's adding to the discussion in a polite way which is what you want. Don't discourage discussion and responses by downvoting them... Upvote the good stuff, downvote hate/spam, leave the rest alone.

You've created a bit of a contradiction here by assuming that the quality of content can be determined objectively in the first place. Quality of content is inherently subjective because there's no definitive "perfect quality." A research paper might be extensive and carefully written, but that doesn't mean that it's better content that a wellcrafted joke- a lot of people would rather hear the joke, which gives it subjective quality. The point of an internet community is to align yourself with others who have similar subjective views on quality. If you want jokes, follow a joke page. If you want papers, follow an academic page. The voting system within those pages determines the quality of posts within their subjective viewpoint.