Main issue is that users cannot modify the order of the categories for themselves:
That means HR, Finance, Legal will see the categories above, 
So if we give all staff access to all categories we also open up confidentiality issues. By creating a group for each category, we know who has access to the information.
There’s also going to be a significant number of categories. It’s far easier for us to have a blanket approach to categories: a group of members, and a group of moderators per Category. The Category is only visible if you are a member of the category group. e.g you only see Docker if you joined the Docker Group.
So basically they just join/leave the group if they want to see the respective category. Hence, a blanket “employees” group wouldn’t really achieve this goal(although we have one anyway for general use)