hmm why do they need water tanks? I thought city supplies the water etc.. Or they are only required for large buildings because of pressure?

NYC underground water pipes are very old and run at low pressure. Buildings above a certain height need to pump water to roof and then gravity feed it.

If you search NYC water tanks on YouTube there are some good documentaries of the companies that build them.

The latter. These tanks are still supplied by the municipal water system but storing it in the tanks allow for consistent water pressure in tall buildings.