The isolation valve switches are used only it a bleed duct leak in L, C, or R duct occurs ( to isolate the leaking duct) or if a APU to pack take off has to be performed; the all 3 ducts are isolated, pack 1 and 3 off and pack 2 is supplied from the APU.
Source: Boeing QRH and OM vol.1, supplemaentary procedures.
P.S.: they are called "isolation valve switch" only, at least from Boeing, the manufacturer.
Cheers