Why a swab sample taken from body sites other than cervix such as oral cavity, nasal cavity be used to detect hpv through pcr?

In general, HPV testing is not recommended at all except in connection with a Pap smear. Nobody needs to alter their sexual life, contacts, etc because of HPV without symptoms -- and half of all people your age test positive at any point in time. Assume all partners are infected and that you are too. Mostly harmless. Get vaccinated to prevent the 9 HPV types that cause most health problems.