There are two companies left in the city that build and maintain water towers, Rosenwach and Iseek Brothers. I guess they don't exist in other cities but here you see them everywhere.
From my roof, I can see about twenty of them standing silently over the city. They are used to deliver water to the upper floors of buildings that are too tall to use regular city water, which is only under enough pressure to go up five or six stories. (Picture a broken fire hydrant.) Buildings pump city water to the roof and it flows down again under force of gravity.
The tanks are basically large barrels made of wood. You have to have them inspected and cleaned once a year and it's probably worth it. A crew climbs up and cleans it out and inspects the seams. The roof keeps dirt and birds out, though I have heard there are sometimes dead pigeons inside, probably one those myths like alligators in the sewers.
