Your shower surround keeps water from coming into contact with the wall behind it.
Tiling a shower wall over drywall.
Traditional cement backer boards and fiber boards require some type of moisture barrier to be.
For most shower walls the drywall installed in the home during the building phase is cut off at the.
While it probably does a decent job the drywall above the surround may suffer from water that sprays up or from moisture that accumulates on the drywall.
Compare this to the common practice of tiling over drywall and you get the idea that you have a totally different shower by using a product that is meant to function in the shower.
Tile adds a layer of moisture protection to drywall.
The wedi board wedi shower system includes special bonding agents referred to as wedi caulk though is much more than caulk that waterproofs the seams in the corners as well as the joints where the 3 5 board pieces join together.