Graffiti is an art form that is short lived. Normally it exists in a busy urban envirnonment. Art pieces get painted over, removed or the weather takes its toll on them.
Marble on the other hand is one of the most durable type of rock we know. It is hard to shape and contrary to spray paint can last for millenia.
The combination of graffiti and sculpting creates a contradiction. An imprint of a snapshot in time. A reminder of a fleeting art form hundreds of years from now.