I can tell you why procurement and also sales category and others can't be changed so easy. A immense number of features inside Dynamics in based on categories, most of all at procurement category.
E.G: Procurement policies as for
- interneal purchase requisitions
- catalog management, incl. Vendor catalog import
- policies for vendor invoice matching
- policies for external Requst for quotations
Furthermore categories are used for handling
- vendor management, supplier rating
- assign product attributes
- ...
So categories (procurement, sales, retail, ....) at product level are used to group products with similar properties and same business processes to provide specific features and processes to these group of items. So it's important to understand and to use categoeries because much business processes are based on this.
