Medicare was designed to increase access to health care and reduce its financial burden on older, retired or disabled Americans. Medicare was never intended to pay 100% of all medical bills, but instead was created to offset the most pressing medical expenses by providing a basic foundation of benefits. Thus, while Medicare will pay a significant portion of your medical bills, it does not cover all services that you might need. Even those services that are covered, they are not covered in full. You still have to pay Medicare's coinsurance and deductibles.
Medicare requires that you pay deductibles, and pays many Part B expenses at 80% of the Medicare approved amount.
Insurance companies sell policies that pay some of these expenses if you are enrolled in both Part A and Part B of Medicare. These policies are referred to as "Medicare supplement" or "Medigap" policies and provide a way to fill the coverage gaps left by Medicare.