Why are there so many advertisements for “add on” insurance targeting people on medicare?

Original Medicare only covers roughly 80% of someone's medical costs and the other 20% is uncapped. For example if someone requires cancer treatment that can cost $100,000 that person could be on the hook for $20,000 that wouldn't be covered by Medicare. This can leave someone with a big GAP in their coverage. Insurance carriers offer Medigap (Medicare Supplement) plans to fill this void. The good news is that the plans are standardized the benefits with one company must match, exactly, the benefits with any other company. The bad news is that, depending on the state, there can be 30+ companies offering the SAME plans at 30+ different prices. Hope this helps!"

Unless you are disabled or have a very low income, there are gaps in the coverage available through Medicare. The exact qualifications for coverage are managed on a state by state basis, so it is difficult to tell you exactly what those gaps may be in your area, or whether you would qualify at all.

With Medicare, you would be responsible for paying coinsurance, uncovered procedures and any excess above your limitations out of your own pocket. If you have many or expensive medical needs, your personal costs could be quite high. In most states, you can qualify for or purchase supplemental Medicare plans, called Medigap coverage, which is meant to directly address the gaps in Medicare coverage. Medigap is managed by private companies, not federal or state governments.

The reason a number of add-on plans are available is because you could left with huge medical bills unless you fill the gaps in your coverage. Typical gaps include coinsurance payments; cost of care limits for some types of care, and excluded services such as the need for blood during surgery. Medigap plans typically provide coverage for longer hospital stays, and may offer as much as a full year beyond the Medicare limit.
If you have Medicare part B, add-on plans cover all of the Part B excesses, including coverage for in-home recovery and preventive care.

Another feature of Medigap and other add-on plans is coverage for all costs after an out of pocket limit. For example, if your out of pocket limit is $4,000, the insurance rider would take over when you've paid that amount, so that you would have some respite from the costs. No matter how you look at it, having the extra coverage is going to save you money if you have any sort of ongoing treatments.
"