Waterproofing under Tiling

Why is waterproofing under tiles and slabs necessary?
Because it is wrong to assume that, for instance,a completely tile-covered wall or floor area is watertight of its own accord. It is true that lining materials and ceramic coverings are resistant to moisture, but the weak spots on wall and floors are always the joints as they allow the penetration of moisture into the substrate, thus causing long-term damage.