Jim,
Initially they were installed as a facility for disabled people and featured a hard floor and fully tiled walls. A drain was usually positioned in the middle of the room, so say, someone in a wheelchair could shower without the need for a cubicle. Since then, they have evolved into more and more expensive creations.
Coincidentally, yesterday i finished the wall linings on mine. One great plus factor from a maintenance point of view, is no need for a shower cubicle, thus no need for continually replacing the bloody silicone around it, even the stufff that is not meant to go black eventually does. Cleaning is a plus also, just point the shower head...job done.