The marble slab is used for cooling English Toffee and for some methods of tempering chocolate. If you have marble or other solid surface counters of similar mass, you can probably forgo the marble slab. The key to the marble slab is getting a big one.
We looked around at several stores, most were too small for pastry, chocolate or the large batches of toffee I make. We finally found one at Macys that was large, but not too thick that it was difficult to lift. We bought it in 1995 and I haven't seen anything like it since. It is only 3/8 inch thick; some thicker slabs may be too heavy to lift. After I pour the toffee from the pot, I lift my slab up at a 45 degree angle in several directions to aid in spreading. You can't do this when the slab is your countertop!