Toxins are released in the body throughout the door. That amount increases during and after work. The water at feeding time helps to flush all that out initially. Since it's so much water to give in one feeding (or 2 if you desire), the excess water continues to flush the kidneys when urination occurs. Couple that with the metabolic water the body produces anyway, and you have a little extra water flushing right alongside the water given in the feed.