When constructing equipment for a space mission, it is common to build a copy of each piece of equipment. This is known as the flight spare. This is built to the same specifications as the original equipment (the "flight model") and can be substituted in the case of damage or other problems with the flight model, reducing launch delays. The extra cost of building a flight spare can be justified by the enormous cost of delaying a launch even by a short amount of time.
The flight spare can also be useful during a space mission if a change to the original plan is required since the effect of changes can be safely tested.
Since very few spacecraft ever return to the Earth intact, flight spares are useful for posterity, and may end their lives in museums.
NASA has two basic types of spares, development spares and operational spares. NASA makes a determination about which parts needs spares based on whether parts are custom built, and the lead-time for procurement; and makes determinations about quantities of spares based on whether the part is critical to system operation, failure rate, and the expected life of the part.