New Yorkers: What are the water tanks on your roofs all about?

When I went to NY I saw little water tanks on the tops of buildings. It seemed so out of place, like little old fashion hats (or farm silos) on top of a modern city.

http://farm2.static.flickr.com/1087/528922019_bdf9e4a77a.jpg

- Why are these necessary? It seems like there is plenty of water around NY.
– How come other cities don’t need these when their buildings are just as tall?
– Do these tanks supply water to entire buildings? Why are they so small then?
– Do all residential buildings have them?
– How does the water get into the tanks?
– why are they made of wood? Don’t they rot easily? Don’t you get woody debris in your water?
– why doesn’t the water leak out if all it is made of is wood planks?
– why are they on tall metal frames?
– don’t they get knocked over and destroyed in storms?
– why do they have ladders on them? do people go inside them?

Any other fun facts you know re: these water tanks are appreciated.