It's also how single-payer healthcare works in the majority of countries that have it.
The problem is simple - in most developed countries, health insurance doesn't bankrupt the people it serves. It also doesn't have policy limits for common terminal illnesses.
So why does this happen in the US - a country with insanely high costs and relatively mediocre health outcomes?
The problem isn't insurance, it's the influence that unregulated privatised insurance has had on health industry costs as a whole.
Hospitals, medical professionals, pharma companies, and insurance companies are all primarily motivated by profit, and will price gouge patients to the maximum extent possible.
Insurance companies may be regulated intermediaries, but in reality many insurance companies are actually owned by hospital chains.
They simply provide a layer of regulated bureaucracy that disguises and legitimises the price gouging.
So while technically it's true that insurance companies have regulated margins, it's also a misdirection. The true role of the insurers is to provide corporate and bureaucratic cover for the outrageous sharking and exploitative profiteering in the rest of the industry.
Customers - i.e. patients - most certainly do not benefit from this arrangement. Shareholders and CEOs very much do.