Opening your CPAM file is the door to French healthcare reimbursements, a Carte Vitale, an Ameli account, and most other admin downstream. APL, mutuelle, and several préfecture procedures cross-check against your numéro de sécurité sociale, so a missing CPAM file blocks more than just the doctor's office.
The legal basis is PUMA — Protection Universelle Maladie. Anyone living regularly in France for at least three months qualifies. The route is the Cerfa S1106 (formulaire 15763*02), filed by registered post to your local CPAM with copies of your titre de séjour, birth certificate (sworn translation if not in French), bail or proof of address, and RIB.
You'll get a numéro de sécurité sociale provisoire within a few weeks, the définitif number a few months later, and the Carte Vitale by post once everything is matched. Reimbursements work from the provisoire — you don't have to wait for the card.