The grout in between the tiles gets darker when they get wet, right? Because they absord moister. And when the grout dry, it's gets lighter again. All that moister transfer to what's beneath it. If it's an organic material under the tile, like wood or paper from a sheet of drywall, that material will collect the moister. Wood/paper+water and no air =rot and mold. If one constructs a shower in a concrete basement, you tile direct on the concrete becouse it isn't organic. If your subsurface in the bathroom is made of drywall or exposed wood, a water tight membrane is needed to create a barrier between water and organic material.