Scheduling flexibility. Sponges can usually be held longer than finished dough.
Increased flavor, developed by the long fermentation of the sponge.
Stronger fermentation of rich doughs. High sugar and fat content inhibits yeast growth. When the sponge method is used, most of the fermentation is complete before the fat and sugar are incorporated.
Less yeast is needed, because it multiplies during the sponge fermentation.