I suppose that I think they used the water and for a lot more than baths. The baths might be a formal and decorative ending, like the fountain where this started.
Maybe “bathing” was more than it is, today. Your clothes were cleaned and the lice were removed. Something like that. Those cisterns probably connected to a lot more than this.
Modern archeology seems to paint ancients as simpletons. I think this work was more justified than just “baths”. That’s all.
Good points. Rome used circa 200 gallons of water per person per day, which is why new aqueducts had to be constructed from time to time. Geographically there was one particular route that had multiple aqueducts built along it at different times.
Getting rid of waste was also a role played by the incoming water. The main sewer, Cloaca Maxima, began as an open trench of sorts, wound up covered in the 3rd c BC by a miles-long Roman barrel vault with other covered passages feeding into it (again, rising population), and the waste got washed out into the Tyrrhenian Sea. Some of it is still in use, take that tree-huggers!