I've had a supplement plan ever since I turned 65 and became eligible for Medicare.
I currently have a Supplement plan. Specifically, it's a Medigap Plan G with Anthem Blue-Cross/Blue Shield. Once you're on Medicare, Medicare typically pays 80% of your medical costs and then you're responsible for the other 20%. If you have a supplement plan, the supplement company picks up that other 20%. The caveat is... the medical need has to be a cost that's covered by Medicare. If Medicare doesn't cover it, then neither does the supplement plan. A lot of people choose NOT to get a Supplement Plan (they choose an Advantage plan instead) because there IS a monthly premium, there is no dental or vision coverage and it can be pricey in some people's opinion. Some people prefer Medicare Advantage. I happen to prefer Medicare with a Supplement plan.
There is an annual deductible (established by Medicare) when you have a Supplement Plan. This year it's $270. Once that's paid, you don't pay any other costs for hospital, doctors, lab, surgery, physical therapy, etc. for the year. There are no co-pays either.