That is a very good question. Section 8 Housing, or vouchers, are typcially designed for affordable housing through rentals only. This excerpt has been copied and pasted directly from the Housing and Urban Development Website, which governs Section 8 Housing...
Housing choice vouchers are administered locally by public housing agencies(PHAs). The PHAs receive federal funds from the U.S. Department of Housing and Urban Development (HUD) to administer the voucher program.
A family that is issued a housing voucher is responsible for finding a suitable housing unit of the family's choice where the owner agrees to rent under the program. This unit may include the family's present residence. Rental units must meet minimum standards of health and safety, as determined by the PHA.
A housing subsidy is paid to the landlord directly by the PHA on behalf of the participating family. The family then pays the difference between the actual rent charged by the landlord and the amount subsidized by the program. Under certain circumstances, if authorized by the PHA, a family may use its voucher to purchase a modest home.
For more information, you can visit the website directly at http://www.hud.gov.