PI AF attribute categories can be used in many ways, but the most visible way is that they are used to sort / group PI AF attributes in PI Vision. If PI AF attribute categories are used in other ways - this can result in confusing attribute groups for the PI Vision end users and duplicated attributes in the list which are confusing.
PI AF Attributes have a "hidden" property which allow those attributes to be shown in PI System Explorer, but are hidden from PI VIsion - this is not security - but convenience de-cluttering.
If PI AF Attribute Categories could have a similar property, then categories created for other purposes would not be used to sort and organize attributes in PI Vision. The default property would be not set - or not hidden.