This post is more of a bleg; because I'm sure someone reading this knows more about the answer than I do.
An insurance company doesn't need reserves if it is insuring a large number of short term uncorrelated risks. The Law of Large Numbers ensures that its annual claims will equal its annual premiums, if it prices its insurance correctly. Every year exactly 5% of all cars insured will crash, so if you price the insurance at 5% of the claim, plus admin costs, you will break even every year.
But suppose there is the same 5% annual risk of a car crashing, but the risks are perfectly correlated. Once every 20 years, on average, all the cars crash. Then you need reserves. If you price the insurance at 5% of the claim, plus admin costs, you would need to keep at least 20 years' worth of premiums, net of admin costs, in reserve. (That's a minimum, because you might get 2 or more crash years in a row.)
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