It's not more useful than hashtags.

It's a hashtag relay.

On the fediverse, there is no central "fire hose", no central source of "every post from everyone". Rather, when someone makes a post, the instance the post was created on looks at the list of people following the person making the post, and sends a copy of the content to all of those instances.

When someone boosts a post, the original post is sent to all instances that have people following the person doing the boosting.

And that's basically it.

So if you're on a single person instance, the only content you will see is the content created by or boosted by people you follow. No other content makes it to your instance, so your "global feed" and your "home feed" look the same.

On a big instance with lots of users, the global feed is just a master list of all of the public content that makes up all of the home feeds of the users on the instance. So the more users, the more content in the global feed.

And when you follow a hashtag, the only content you see is hashtag content that made it to the global feed. So if no one on your server follows a particular person, none of their hashtag content will make it to your instance.

Relays are a fire hose. Every instance that adds a relay sends all of its public content to the relay. And in turn, the relay sends back all public content it receives from every instance subscribed to the relay. So if you're a single user instance, and you subscribe to a relay, your global feed will look very different to your home feed, because the relay will be sending you content from all of the instances subscribed to that relay!

tags.pub is a garden hose instead of a fire hose. It's a way of getting filtered content from a relay, so that only content that matches the hashtags you or your admin have subscribed to make it to your instance. The other main difference between tags.pub and a typical relay is that individual users can subscribe to a tag on the tags.pub relay, and get content from it even if their instance admin hasn't added it as a relay.

If you run a small micro-blog instance, you follow some people and maybe some communities. Tagged posts come only through these follows, so you only see a tiny portion of all tagged posts.A hashtag relay tries to aggregate tagged posts from the whole network. By following a hashtag on such relay you can see more posts on the topic that interests you.