If you have comprehensive and collision insurance on your owned car, your carrier also pays for your common-law responsibilities on a rental vehicle. The controversy arises when the rental company decides that you signed for contractual responsibilities that exceed your common law obligations.
Here are several examples. A car that can be fixed for less than its current value is generally fixed, not totalled. Some rental companies make it their call as to when a car is totalled. So if the car can be fixed for $5000 but they decide to sell it at a net loss of $8000. They charge you $8000 and your carrier pays $5000 but they are not likely to pay or reimburse that difference.
Suppose they fix the car and your carrier pays to fix it. When they sell the car later down the road, they must declare that the car was in a major accident. What does that do to their resale value? It diminshes the worth of the car. Most carriers do not pay for diminished value unless required by a few state laws.
Suppose you’ve parked the car and a tornado or hail storm sweeps down. In most cases you are not responsible for things you had no control over. Your comprehensive would still pay for the actual damages to the car. But what happens when the rental car wants to be reimbursed for the loss of rental time until the car is repaired or replaced? Most carriers will pay loss of use if you caused the damages. Most will not if you did not directly cause the loss and the law does NOT hold you legally responsible.
Taking the waiver is the safest way to rent a car. It waives the contractual AND common law obligations for all damages to the car. Otherwise they can and will attach your credit card.