This is a way to save water for those who like taking baths.
Usually, when you fill a bath, you do so to a required depth, which necessarily means that the other dimensions of the bath tub space has to be filled also.
Or, if you want to save water, you have to fill it less, defeating the purpose of a bath altogether.
By inserting moulds which cut out the extraneous space in a fixed dimension bath tub, only the volume of the bath which accomodates the occupant is filled, allowing the desired depth to be achieved, but not having to fill the base area all the way up to that depth.
Leg shaped moulds for tall persons can be made, as well as leg and back sections for smaller people, and even small child size moulds, entirely suited to the size and volume requirements of the individual user.