jordanjay29 just better visibility and management. Let's say I have a tag call "Film" there could be "Action" "Drama" "Musical" etc. under "Film"
However, "hiding" the sub tag under "Film" untill the user click on it could cause them to lose interest. Suppose I'm trying to promote my community to people that love "Sci-fi movie" by running ads of my website and articles. It would be easier for people to lane on the home page if they can see the "Sci-fi" sub tag.
If we have multiple primary tag (one for each genre), we are run into the risk of fragmenting the user based. People that clicked in one genre will less likely to click on another genre. Also it's not really scalable as you add more genre.