I am covered, during my time I am diagnosed with cancer. I get treatment, and am declared "free" of cancer. I remain that way for 5 years, and claim no more benefits. I am still covered.
I leave, and join a new company. To enter their health insurance, I have to declare any "pre-existing" conditions. They see that I am in remission from cancer, and offer my employer two choices: pay hugely inflated premiums, or get no cover. My new employer never expected to need to cover premiums 20 times that of their existing employees, and consequently I am now screwed.
Group plans are limited in how long they can exclude someone for pre-existing conditions, and guarantees no waiting period if they already had qualifying insurance.
The private market doesn't have the same protections, for reasons both good and bad.
