The organic cotton absorbs excess milk and helps to avoid visible leaks.
I liked to use one of the silk/merino wool pads closest to the breast and then layer a cotton one on top for extra leakage protection. That way, I was able to reap the benefits of having the healing properties of the silk next to my skin, the natural absorbency of the merino wool and then the extra protection from the cotton. This was especially helpful in the early days when everything was calibrating and my leakage was heaviest.