Marble slabs are used as stable, solid, hard surfaces to pound leather on. I have two pieces of marble, a 6x12x2 inch thick piece for heavy pounding like hardware and 3-D stamps, and a thinner 12x12x1 inch slab for all around tooling and smaller stamps and hardware. On the back of the marble I have adhered a piece of "poundo" board. Poundo board is actually just a thick slab of rubber used to help muffle the noise of all the pounding. Poundo board is also used under tools and anvils to protect the tools (like chisels that punch right through the leather) and to protect the marble from being nicked or cracked by the steel tools.
You're not a leatherworker unless you have at least one slab of marble around to pound on! ;-)