Parallelism can be used to ensure that all the points on the considered feature lie within a tolerance zone oriented parallel to the specified datum, or that an axis of a considered feature is parallel to a datum. Parallelism is generally used As a refinement of location or size.
Let’s take an example of a gear that has two faces bearing axial load. To have even contact on the sides, the two surfaces must be held parallel. To do this, we can either control the width of the gear (distance between the two surfaces) more tightly (and therefore, potentially more expensively), or we can use parallelism to refine the orientation of the surfaces without tightening the distance (size – thickness) tolerance. This type of control will let you open up the width (size) tolerance while still retaining the necessary orientation requirement, and thus avoid rejecting functional gears.