Car rental coverages can be SO Confusing! I work for a MAJOR car rental company. We offer 4 different coverages that are for 4 seperate things. And you are right, most people do NOT need extra coverage from the car rental company but there are some who do ! The costs are VERY high but you wouldn't want to get stuck paying for the total value of the car!
Loss Damage Waiver/Collision Damage Waiver- Covers only the Car Rental Company's Car for Loss, Stolen or Damage to the vehicle. It DOES NOT cover someone elses car (EG. If you hit someone else...it doesn't cover their damage.) If you have your own insurance for your own car, check with your insurance agent to make sure that you are covered on anything that you drive. Some but NOT ALL credit card companies will cover you as well BUT only on certain cars for certain lengths of rental with some deductibles. THERE IS A LOT OF FINE PRINT !!
Personal Accident Insurance - Medical Coverage and Accidental death and dismemberment. If everyone in the car has medical coverage, this would be in addition to what you already have. It covers the renter 24 hours a day, in or out of the car, and then covers anyone traveling inside the rental car.
Personal Effects Protection--Covers the contents of your vehicle.....way too many exclusions to be worth it. (Can't claim that you had your 4,000 dollar computer in the back seat...)
Additional Liability Insurance--Every state has different minimums for liability.....usually they are quite low. The car rental company gives you the state minimum based on what state an accident happens. (If you rent a car in Georgia and drive to Florida and have an accident in FL then you get the liability limits for FL)....If you purchase the ALI then you get 1 million dollars worth of coverage for injury and damage ......
This of course, is the Reader's Digest Version! I am sure that if you go to any of the major rent a car web sites, everything is spelled out pretty clear.
Hope this helps a bit !