There isn’t much about tags in the standard Discourse documentation because how they are used is up to each site. I think the original intent of Discourse was to actually use tags instead of the categories/forums that we have. In any event, that is definitely not how we use tags here.
Tags are just a way of categorizing things. For example, we use the Breaking News tag in MPSIMS, which flags the thread as Breaking News and therefore the specific Breaking News rules apply. Similarly, the Factual tag is used in QZ which means that the particular thread uses FQ rules for fact-only type discussions instead of the normal QZ rules which are more like IMHO/MPSIMS.
The vast majority of tags are used in CS. The intent is that if you are interested in something, like say things on Netflix, you can click on the Netflix tag and see all of the discussions about Netflix shows and movies. There are tags for Chess, Golf, Trivia, all sorts of things. In the case of streaming shows, many of the services already have tags set up for them. If you see a discussion about a show/movie that doesn’t already have a tag set up, report the OP and let us know what tag needs to be added.
In the case of streaming shows, the tag will show you what service the show is on, so that’s an additional benefit for that type of tag.
These aren’t supposed to be sneaky “in the know” sorts of things. We do actively encourage people to use them. While there isn’t much about them in the standard Discourse documentation, each forum that uses them does have some notes about them in the forum-specific rules for that forum.
Tags do not show up on the main SDMB page. But they do show up on the main page for each individual category (CS, FQ, IMHO, etc), directly under each thread title. And when you open the thread, the tags are again directly under the thread title, and remain under the thread title in the ribbon at the top of the page so that you don’t have to scroll up to the OP to see them. The tags are always visible, and you can click on them from anywhere in the thread.
ETA: @What_Exit very kindly volunteers his time to go through new posts and adds tags as needed, as do some of the other mods, so if the OP forgets to add a tag it’s no biggie. Again, if you notice something that was missed, just flag the post.