Three henchmen returning coal to a nearby Workshop.
The Workshop is a structure that serves as a collection point for coal. While Henchmen normally bring their gathered coal to the Lab, travelling to and from a coal pile that is far away from the Lab is inefficent and potentially dangerous. A Workshop solves this problem as henchmen can build one close to the coal pile and deposit it there instead.
Since Insect Invasion, Henchmen can also spawn from a Workshop. If the Lab's rally point is placed on one, newly created henchmen will appear at the building, allowing them to get to work straight away.
Also since the advent of Insect Invasion, a Workshop allows the player to convert coal to electricity and vice-versa. But you won't get 100% value whenever you convert a resource; for example, 50 coal is converted into 15 electricity.