They are purely to manage your rules what can be useful if you have many of them. With only one firewall you can completely ignore the separation of "mandatory" and "default" and put your rules anywhere you want. These are meant to build hierarchical policies for multiple devices. But what about one category for incoming traffic and one for outgoing traffic? Or if you have a DMZ and put these rules in an extra category?
With only a handful rules I would not care about the categories. But when your rules go to the hundreds or thousands, you will likely use them.
