Commercial insurance only pays the rest with social security. Why?

Hello, when commercial medical insurance is reimbursed, if there is social security, the rest of commercial medical insurance will be reimbursed after social security reimbursement, but it cannot be reimbursed repeatedly. If there is no social security, it will be directly reimbursed by commercial medical insurance. In addition to medical insurance, critical illness insurance and social security do not conflict, and the insured amount is paid.