Traditional Shower Pan Design
In the drawing above, the PVC waterproofing liner is below the mud base. This mudbase is typically 1" thick at the drain and slopes up to the edges of the shower which is several hundreds of pounds depending on the size of the shower. This is the blue colored area.
It is a common misconception that the tile is the waterproofing for the shower but this is not the case. Although the tile does direct the majority of the water directly into the drain piping, the grout and other breaks in the tile (wall edge and at the drain piping) do allow water to "seep" into the mud base below. The mudbase absorbs this water and probably never really dries out. This is an ideal place for mold and mildew to thrive.
For wall preparation, depending on the age of your shower, it is either a wire lath which has been packed with mortar to form the wall -or- a concrete backer board. Both of these products will hold moisture just like the mudbase providing the ideal environment for mold and mildew.
Schluter Kerdi™ System
In this system, the waterproofing membrane is above the mudbase and the tile is bonded directly to the membrane. Just as with the traditional shower pan design, a small amount of water still seeps through the grout lines and gets underneath the tile but is immediately directed towards the drain piping. Thus, the mud base cannot become saturated with the water.
For wall preparation, I use a backerboard which has a waterproof membrane on the surface of the board. Any water which finds its way through the grout lines stops right there. Thus, minimizing the environment for mold and mildew development.
Another way to minimize the seepage of water through the grout lines is to choose an epoxy grout vs traditional cement based grout.