I have a cat who doesn't always urinate in the littler box.  I need a waterproof backed rug to protect my carpeting around the litter box.  The rug soaked up the urine and the thin sheet of plastic glued to the back of the rug did not stop the urine from wicking to the carpet  at its edges.  It was also hard to see where the cat urinated (to clean it up) because it was absorbed so quickly.