In addition to that, most personal car insurance policies cover rentals, and every credit card I've ever seen provides insurance for cars rented with that credit card. The average renter is covered by three different policies (the rental company's policy, their personal auto policy, and their credit card, although they don't perfectly overlap and may not cover everything you'd want) before the rental company tries to get them to buy a fourth.
Please do your own research before relying on any of this, of course! Especially since individual insurance and credit cards may vary. But personally I've not seen any reason to purchase the insurance they offer.
Credit cards generally only cover the cost of the rental car, I've not seen one that provides liability coverage. So if you don't have a vehicle of your own, which would provide such coverage, the little-known rental car policy is what saves you from having to fork over extra cash for liability coverage.
Please do your own research before relying on any of this, of course! Especially since individual insurance and credit cards may vary. But personally I've not seen any reason to purchase the insurance they offer.