Bill wrote:

It seems if you were installing anything that has to do with water
(bath, kitchen, a boat)
then you would desire certain surfaces to be level. (To me), it
seems that anything
else should follow the wall or ceiling lines or be balanced in
between them.

Not always. If a bathtub or shower pan or drainboard (do they call them that
any more?) should or could have a slope to facilitate run-off.