We recently discovered this feature of the platform. We already knew that we can set watching or tracking of entire channels or topics.
The TAG watching brings it to another level because that way, you can watch specifically for the matters that interest you most across any channel, topic or post.
To watch a tag follow the procedure described below. Although you can only select tags that already exist, nothing prevents you from creating an interesting topic on the matter, and tag that topic with your choice of tag which will then be aggregated to the tag list.
Although it is most efficient if topics are specifically tagged by the topic's author, Discourse gives us a little help by also indexing words in titles and some important words in the posts.
