This article describes a feature in the first Apocalypse Rising
The Shed is a common building you see often in urban and industrial areas. They spawn civilian loot, items ranging from car parts to foods and drinks. Like any other building it can be used to make a base despite its small size.
Sheds were presumably, before the apocalypse, used for storing car parts and other specific items. They were then left near houses and barns. They are usually found in small hamlets, along with other buildings, and in industrial areas.
They are known to spawn all parts needed for vehicles so if you have a vehicle, be sure to check these sheds.