It is important to expell the toxins from your body through all the organs. Many of the toxins leave your body through using the bathroom (kidney, liver, gall bladder, intestines, colon etc.) but there is more the body can do and that is to push out "stuff" through the skin (pores).
Im sure there is a much more scientific explanation than that, but I will say the more ways you get the gunk out the sooner you'll feel all the benefits.