There are gaps in Medicare coverage. There's long term care that isn't covered by Medicare; people can self insure, or purchase separate coverage, or many rely on Medicaid which requires exhausting assets first.

I've watched both my parents go through this, with significant chronic health issues. They had good private insurance on top of Medicare, they routinely had $10-30K annual itemized deductions for health related expenses that were not covered by their insurance. And that was with a daughter who is an MD who invested a massive amount of time and effort to get insurance to cover as much as possible.