Grouping courses for the public listing.
A category grants nothing
A category is only a way of organising and finding. It does not change price, does not grant access, and plays no part in enrolment.
Deleting a category does not delete courses
Removing a category leaves the courses alone and simply takes them out of that group. But that category's page no longer exists — if you linked to it anywhere, the link breaks.
A course can be in several categories
There is no limit. A course in no category at all still appears in the overall course listing.