Sponging, along with ragging, is one of the basic techniques used for many faux finishes. Careful thought goes into selecting sponges, different types of sponge give different finishes.
The holes in the sponge created a speckled look, allowing the base layer to show though. This is necessary for imitating stones like granite.
Ragging is another basic technique, which can be used with or instead of sponging. You can see more on that here.