Dave:
Before the wide availability of natural gas a city would make its own 'coal gas'.
The gas was manufactured from coke in big furnaces called coke retorts then piped into in a dedicated building called a gas plant for cooling and cleaning (Walthers makes an HO scale version called Empire Gas) then pumped into the gas holder and stored for delivery to street mains mostly for gas lamps at that period in time. The gas holder was a telescoping tank. The one in the picture has three sections. As the tank filled with gas each section in turn would raise until completely full. The superstructure you see on the exterior is a support structure for the tank sections as they were raised or lowered.
Both the gas plant and gas holder are shown below (Walthers)