How to Use Categories?

I'm finally understanding the hierarchy of Classes, Courses, and lessons. Now come categories. I see that categories are a taxonomy term. On my own websites I use Taxonomy terms a lot, so I understand what they are and how to use them. The Opigno User's Manual tells how to manage categories (which I already know how to do), but doesn't say much about how to use Categories. What I don't understand is how they are used in Opigno. I see that they are attached to Courses. However, in a sense, the names of the courses (i.e. Science, Math, English, Writing, etc) are already like categories, so I imagine I wouldn't use those types of names for my categories. If I were to attach a category to a "Science"' Course, what type of categories would I use? Could you give me some examples and use case scenarios? The manual does say that Courses can be filtered by category in the training Catalog. But what terms would I filter them by? Would I filter the courses by grade level (i.e. 9th, 10th, 11th), or what? Besides in the Training Catalog, where else are categories used? How are they used? What type of reports are they used in?   Thanks, Best Wishes, Stephen