The cat's nails can cause holes in them which will allow urine to leak between the liner and the plastic bottom of the litter box. (catbehaviorassociates.com)
Most cat litter is made from clay, which sticks to the sides and bottom of the litter box when it gets soiled. (worldsbestcatlitter.com)
When I change them I sprinkle it in the bottom of their litter box and on the floor under the box. (simplypetsmagazine.com)