Whenever two surfaces or features need to work in sync with each other and constant distance must be maintained, parallelism is effective. Whenever you have a part that must always fit nicely between two planes that need to reference each other, it comes in handy. Even though surface parallelism seems to ensure that a flat surface is mating with another flat surface, it can also be applied to two sides of a hole or cylinder to avoid a taper. Any part with two critical flat surfaces such as gears, would call out parallelism.