Not only is it a rule, it's also a measure of security. Each rabbit should have a unique tattoo, and in case your rabbit is stolen or lost, the tattoo number can aid in finding the rabbit better than its name or breed/color.
In a class where hundreds of rabbits of the same breed are being shown, things can get pretty mixed up. In a class of Florida Whites, where the only allowed color is a white rabbit with red eyes, there's almost no way to tell the rabbits apart. That's when a tattoo really comes in handy.
One time I even got Cinnamon mixed up with another junior doe her exact color. They looked almost identical. Good thing they both had tattoos! Even though, when I looked closely enough, I could see my Cinnamon. She was the one trying to get out of the coop and jump off the table. lol!