In our shower we poured a mortar bed with a preslope, added a liner, and then poured mortar on top with a slope towards the drain. But it started me wondering what the point of the top slope was, when using a self leveling bed mix would be much easier for the average DIYer (including me for my next shower redo). Given that any water making it through the tile and mortar will just be directed to the drain weep holes by the liner on the preslope, is there any point to the slope other than to prevent standing water on top of the tiles?
The slope exists for draining. The mortar bed of the showerpan creates the substrate on which tiles will adhere and create a physical/mechanical bond. The several layers of foundation (frame, liner, mortar bed, etc.) provide protection from water damage to the frame and foundation of the structure. Standing water in a tiled shower will create problems for the installation, a medium for bacteria, fungi and viruses, slippery footing and the slope is required building code.
The tiles don't have to be sloped but they must keep the slope that existed.