Is it permissible to benefit from the insurance that the employer pays monthly, which allows the employee and his family to benefit from free medical treatments and services, knowing that it is annual insurance from which one benefits whether harm occurs or not?

If an employee is entitled to benefits and services by virtue of an employment contract, and their employer refers them to an insurance company to claim their right, then there is no blame upon them for claiming it. The sin, if any, is borne by the employee's employer, not the employee.