The gasometer was the device that held the gas after it was produced and before it got piped out to the customers. This is perhaps the most noticeable structure on the Gas Works site because it is several stories tall but its purpose remains a mystery to the observer. The gasometer worked like a collapsible cup that might be used on a camping trip. There are large cylindrical rings at the base of the structure (thus why the ground is raised below the gasometer) and a metal lid on top. When enough gas was produced, one of the cylinders would rise out of the ground, and when the gasometer was full, the whole collapsible structure would rise up to the top of the steel frame.
On a pointer for reddit/r/rva, here is is a short video about gasometers: