A heat spreader is most often simply a copper plate, having high thermal conductivity. Functionally, it is a heat exchanger that moves heat between a heat source and a secondary heat exchanger whose surface area and geometry are more favorable. By definition, the heat is "spread out", such that the secondary heat exchanger increases the heat capacity of the assembly. These properties make it a better match for an air heat exchanger, since the low heat conduction for air in convection is matched with higher surface area.
A heat spreader is generally used when the heat source tends to have a high heat-flux density, (high heat flow per unit area), and for whatever reason, heat can not be conducted away effectively by the secondary heat exchanger. For instance, this may be because it is air-cooled, giving it a lower heat transfer coefficient than if it were liquid-cooled. A high enough heat exchanger transfer coefficient is sufficient to avoid the need for a heat spreader.
The use of a heat spreader is an important part of an economically optimal design for transferring heat from high to low heat flux media. Examples include: