Tanking is used to create a non permeable membrane between your tiles and the surface substrate. The technology of todays adhesives and grout and in some respect the tiles, means that all three will allow water to penetrate at different rates in a wet area. ie shower / bath location. To combat this they also allow the penetrated moisture to dry back out too. Unless they have further to penetrate - ie the wall surface. The membrane will make the barrier to direct any penetrated water back out to the surface as it has nowhere else to go. When tanking is not in place, with a variation of time brought in by the amount of shower use etc. The moisture will travel down within the wall behind the tiles and create the unsightly black spores that will then travel back up the tiled wall. Resulting in damp areas, loose tiles and smell etc. Tanking allows a guarantee for up to 10 years that these spores will not appear.