Why are primary care physician considered gatekeepers

PCPs (primary care physicians) are gatekeepers because they treat the while patient and manage referrals to specialists, rather than a patient deciding which specialist's opinions are sought. this concept became popular with the adven advent of HMOs - and services would only be paid if the PCP sought approval for major diagnostic tests and referrals. PCPs were financially rewarded if their patient population had fewer expensive tests and/or referrals.