Rocks probably help drainage.
I’m not sure, but coarse old fashioned cat litter (NOT the clumping kind!) might work the same.
Maybe someone else can offer suggestions.
I’m reading on the web and it appears I need screen in the bottom to keep soil from going outside the container and then rocks on top of that to keep soil from clogging up the drainage. If I had screen and I’m not positive I don’t, will have to look, I have marbles that were used for decorating and they are just in the way now, so could use those. They are too small to use without the screen keeping them from going through the holes.
Rats, why does this have to be more complicated than putting in soil and plant? Rats.