One could argue about what defines "useless" indefinitely as it's a matter of choice.

In the end, admins decide if content is useless, unless they give their power to the moderators (happens on reddit). Moderators decide if content is useless unless they give their power to users (occasionally). The majority of users don't vote based on quality/content/on-topic discussion/etc, but instead on agreeability, access (imgur.com vs. any other image site), and digestibility.

Take a power user like /u/apostolate or /u/alpharedditor (two recent power users who will most likely continue to rise over the next few months before burning out). Their comments are generally short, unoffensive (to the reddit community), and are close to what everyone else is thinking. These kinds of comments are easy to read and digest so that users immediately know how they feel and vote accordingly. The algorithm for voting on reddit stacks the website to favor this kind of content because of the attitude of its userbase, and it does so for comments and submissions alike. The problem is that these comments and submissions don't add much content or value to the site. Sometimes they make people chuckle, but often, the top posts on reddit exist because they're short and agreeable, something we often (as humans) misrepresent for humor.

Now, you may disagree with me and find those comments/submissions wonderful, I find them useless. This site, which was originally intended as a news and blog link-aggregator has gained its own culture that's been outputting simple content for a couple of years now. Even on news links, I often have to search past jokes and one-liners to find any discussion, and it seems that anything that isn't the popular opinion on the site gets downvoted, hiding it. But the worst part about it is that millions of users all think they're part of some super secret club, so they act the same way in /r/funny and /r/trees as they do in /r/news and /r/askscience, and it's such a large group of users that it spreads so quickly, which is exactly why 4chan refers to this mindset as cancer. I don't find it useless because I don't like it, but because it adds little and hurts what the site was intended for, making it impossible to have a serious aggregator with real discussion and a decent content stream at the same time.