 for waterproofing and moisture proofing the area behind that tile. And it goes quicker. If you have to sit there, put in the backer board, tape the seams, do all that, put on two or three coats of Redguard and deal with that smelly mess, why don't you just do it all in once? Put the foam board up there, the backer, tape the seams, the screw holes, call it a day, you're done. So much faster and you have a much better system.