Lets say you payed for insurance, but were healthy for the 90s. So ~10-years of paying insurance. Around 2005, you are diagnosed with Diabetes, so your insurance pays for your care.
Around 2007, the recession happens and you lose your job. You immediately find a new job, but you need to get new insurance. Diabetes is now a "pre-existing condition", so your insurance no longer covers your condition.
Its important to recognize that changing health-plans is a regular event, that shouldn't punish people with chronic conditions. Diabetes is lifelong, once you get it, you live with it for the rest of your life. Insurance companies don't want to pay for that kind of issue. With all talk about "Repeal and Replace", its important to recognize the tradeoff.
As long as "pre-existing conditions" are banned in the USA, I don't think its a big deal to lose employer sponsored health care (at least, compared to pre-Obamacare). We should build a world where it is easy to lose (and regain) health coverage... compared to the pre-Obamacare world where it was a big risk to lose coverage. Hopefully laws of the future make it easier to switch health care plans without problems.