Because Original Medicare pays for only about 80% of your coverage.
Medicare Supplement (also known as Medigap) covers things that Original Medicare doesn’t, like copays, deductibles, and coinsurance (Need to know more about copays, deductibles and coinsurance? Learn more here). It does all this with a steady, predictable, monthly bill you can budget for. And it can’t be cancelled. It will be renewed for as long as you pay your premium on time.