I you are making a box, you can start with the blank and pocket out everything inside. Or you can make it out of layers. The layers that are just the sides don’t need a pocket, you can use a hole. Cutting the hole is a lot less machine time than cutting a pocket. Then you glue the layers up to make a box.
It’s also useful for something like a sign or logo. Pocketing out 24"x36" is going to take years. But cutting parts out and glueing them together is an hour.