Why is the water tower cylindrical?

Easier to manufacture than other tower shapes.

It has a relatively small specific pressure on the ground where it is built, because it is distributed over the area of the circular base of the tower. The circle is the maximum area given the dimensions.

At a given height of the tower (usually not higher than the last floor of the house, where water is delivered from the tower), there should be a relatively compact capacity and at the same time, spacious, which is precisely what the shape of the cylinder contributes to.