The Department of the Air Force (as a good example) hires temporary contractors to work alongside FTE's. When the FTE's become proficient in the area of responsibility, the contract ends. This provides greater responsibility and opportunities for the FTE. The need for year to year contracting because of the lack of existing experience eventially gets significanlty reduced or eliminated.
Additionally, the FTE learns a new skill and is able to apply it more effciently with the agency culture.