The main reason to use sulfites at the beginning when you mix the must to kill wild yeast and bacteria.
After fermentation starts, the main reason to use sulfites is as an antioxidant (sulfites bind with the wine so oxygen can't) for racking and as a preservative.
I use sulfites, in much smaller amounts than commercial wines have, for the antioxidant and preservative qualities at every other racking and at bottling, with a goal of keeping the sulfites around 50 ppm.