What are the big brick circles on San Francisco streets?

I've figured out that they are called cisterns. That they are cavities that hold between 10,000 and 100,000 gallons of water. I think I understand that there was a relationship to these being created, and the 1906 earthquake. So, to be more specific, are they still maintained? Do they still contain water? Is it potable water? Is it only for fire fighting uses? Are they obsoleted because of hydrants? Are they still used?