If medigap policies only cover medicare approved services, and medicare pays for those, then why do we need medigap?

Medicare and Medigap cover the same items but Medicare covers only UP to 80%, or less. The private Medigap policy covers the remaining unpaid charges. If you have a $10,000 procedure, you pay $2,000 or more without a Medigap policy. If you get an Advantage plan, that includes Parts A,B,D, and you cannot even purchase a Medigap because theres no need. Some locations have plans with zero premium. My premium is $110 a month for an HMO Advantage plan. It is the only Advantage plan where I live. Another benefit is my Advantage plan has no Part A annual deductible which is around $1200 right now.

To help pay for what Medicare doesn't pay. That's why they're inexpensive...relatively speaking.

Medicare pays 80% of approved services, medigap pays the other 20%

Plan G has a $183 Part B deductible that you must pay first.

After you pay that deductible the plan will pay everything that Medicare does not pay. For example, Medicare has a Part A per occurrence deductible of $1340 that your Supplement will pay instead of you. Medicare has a $335 to $670 per day hospital copay after a certain period of time that your Supplement will pay. Medicare has a 20% (up to 50% for certain procedures) Part B co-insurance that your Supplement will pay.

If you want something that gives you additional coverage (such as dental and vision) try a Medicare Advantage plan.

medicare pays maybe 80% of those. Without a medigap policy or advantage plan, you pay the rest.

Because Medicare doesn't pay 100%. There's ALWAYS "coinsurance". Medigap helps pick up the parts that Medicare doesn't pay.

Medicare has many out-of-pocket expenses. Have you ever heard of the donut-hole? You really need to learn about Medicare.

Medigap pays the deductibles & copays. Medicare does not pay in full, but no more than 80% of allowed charges. E.g bill is $1000, Medicare allows, say, $800 of that, and pays, say, 75% of the $800, then Medicare is only paying $600 out of $1000. You pay the balance, or get Medigap to help pay the difference.