It can be. Rubble is just some small pieces of live or dry rock. When added to the fuge, it increases the amount of rock you have for bacterial population with the added benefit of a place for pods to collect and multiply. Some of the pods will find their way back into the display.
Some people use ocean mud instead or in addition to the rubble. It is really up to you but you may want to start with just the macro algae to keep it simple. As your tank matures you will find opportunities to improve your tank. The fuge is one of those places where you can put things that you want to protect from the main display or to protect the main display from them. It also gives you more flexibility for things like ocean mud or pod havens.
The main thing idea is that the macro algae helps with nutrient export. There are other things that you can do with a fuge down the line. A fuge is not necessary for a tank but is a good low tech alternative to many other export mechanisms.
