Medicare Supplement Plans, also called Medigap Plans, fill the gaps of what Medicare does not pay. The better the plan the more gaps it fills. Plan F, for example, fills in where Medicare leaves off, leaving you with no copays or deductibles. Where as with Plan N you have copays and deductibles, and it does not cover Part B excess.
Please note: Medicare Supplement Plan F and Plan C will be phased out. If you don’t qualify for Medicare until January 1, 2020 or later, you won’t be able to buy Plan F or C. This change applies to Medicare Supplement plans that cover the Medicare Part B annual deductible (Plan F and Plan C). If you qualify for Medicare before January 1, 2020, you may still be able to buy Plan F or C. If you already have Plan F, you can generally keep it. The same applies to Plan C.
The reason there are so many plans to choose from is because not everyone wants the same thing. Some people are willing to absorb more cost in exchange for less premium. Some are middle of the road.