I think it was just the style. Mine is like that too, from 1957. There is definitely no function for it in my bathroom.

The bath was put in the same time the upstairs bath was built. I think the plumber just ran a horiz drain pipe and the carpenter built the drop ceiling. The PVC pipe could easily been raised 2-3 inches so ceiling above shower could have been flush with bathroom. I am redoing a similar bath in another house and am using this one as a reference. I am tearing out a small shower stall and putting in a larger one. I have the same ceiling height issues to deal with. I have a low ceiling. 6'-6" and do not want to give away 2" of height over shower

Functionally, I have no idea. Stylistically, some designers will use a drop ceiling to highlight an area, set it apart, or make it more cozy. But this usually works best when the ceilings in the room are so high that they cry out for differentiation.