Here we see the bench seat and shower pan hot mopped with tar. That area is subject to moisture the most, and needs the intense protection provided by the tar mop. Above that area isn't so problematic since water will roll down off the wall and into the shower pan.
The tar offers the best protection from moisture intrusion but the greenboard offers sufficient protection at a fraction of the cost above the most moist areas - the shower pan. Using these different water sealant barrier options gives the best protection where needed at the lowest cost.