Junk on the intestinal wall keeps digestion from functioning properly, so gotta clean the bowels out.
The liver filters the blood and needs to be cleaned out, too.
It all works together. Get everything cleaned out. Lymph, too, because stuff gets deposited in there. Exercise helps pump stuff out of the lymph system because it doesn't have a pump like the heart. See Doc's order of cleansing and that will explain why.