Assigning roles to users in every category may become cumbersome and unmanageable. The concept of role inheritance solves this problem. Put simply: if a user plays a role in a category, then the user automatically plays the same role in all it's sub categories.

Continuing from the previous example, if Mark is the worldwide sales director, then if he is assigned that role in category "Global > Sales", then he automatically plays that role in categories "Global > Sales > US" and "Global > Sales > Europe" as well. There is no need to assign him to that role in the sub-categories.