Tiberium Refineries process raw Tiberium harvested in the field, turning the crystals into valuable credits for use in the GDI war effort. Expensive and power-hungry, they are nevertheless central to most GDI bases. Refineries can hold a finite amount of Tiberium. Refineries return 500 (750 in Kane's Wrath) credits when sold.
It is possible to sell the refinery and just use the included harvester instead of the building itself, in order to collect resources faster without tying up the first War Factory produced. Usually the Refinery is able to support multiple harvesters, as it will have the docking space to cycle between unloading at least two harvesters; so this is a good use of the MCV's build time while gathering more resources to advance in building tier towards the Command post.