For example, if you go to a doctor who's not in your health insurer's network, you're probably not covered. In some states, you can buy a "full coverage" (liability + comprehensive + collision) policy with a liability limit of $10k, so if you hit someone with your car and they have more than $10k in medical bills they can sue you for the balance. Your provider network and liability limit will both be spelled out explicitly in your policy documents, and neither concept is really that complicated, but if you don't know about them you could be in for a nasty surprise.

Of course, all of that creates a giant pain in the ass for consumers, and that begs the question, why doesn't anyone just make an insurance policy that covers everything the policyholder thinks is covered? And the answer is that nobody would buy it! It would be more expensive (usually much more expensive) than its competitors, and people are generally very price-sensitive in their insurance purchases, and no one would read the fine print to see what they're actually getting for their money.

P.S. The rabbit hole of stupid insurance regulation runs deep. Texas county mutuals are a fun example. Captive reinsurers are another.