Most petcocks have three positions: In most cases, the main and the reserve settings are actually drawing from the same tank, but there are two outlets through which the fuel may leave.
When the selector is on 'reserve', the lower outlet will be used, which allows all or most of the fuel to be drawn from the tank.
Because in most cases the 'reserve' setting simply allows access to all the fuel, rather than a dedicated tank, the 'reserve' selection allows the rider to deplete the main fuel supply as well as the reserve.
The range of an aircraft usually includes an allowance of fuel in reserve in case of a diversion from the planned landing point due to inclement weather or other problems, in which case the reserve equals the fuel necessary to fly to the divisionary airfield.
An adequate reserve can sometimes be treated as an unnecessary luxury in military aviation, but civil authorities consider adequate reserves to be a vital necessity in air transport operations.