When Medicare was implemented it was determined that there should be balances to the care that people received. This causes people to use Medicare benefits wisely.
There are 10 standard Medicare Supplement policies on the market. They are also known as "Medigap" coverage. These policies pay the co-insurance amounts that Medicare does not pay. For example on days 21-100 that Medicare pays everything except the $101.50 - a Medicare Supplement policy could pay that. (Medicare Supplement plans A and B do not cover this co-payment for skilled nursing facilities)
If Medicare is not paying for care or services, then the Medicare Supplements will not pay either. There are a few exceptions to this.