The rental counter presents a coverage decision under time pressure, with a queue behind you. The difficulty is not the price but the fact that three separate systems may already cover parts of the same risk.
The counter product is a waiver, not insurance
What is usually sold as collision damage coverage is typically a waiver in which the rental company agrees not to pursue you for damage to its vehicle. It is a contract term rather than an insurance policy.
That distinction matters because a waiver responds to the rental agreement's own conditions, and those conditions can exclude certain uses, drivers, roads or vehicle types.
Separate products covering liability to other people, and personal effects, are generally offered alongside it and answer different questions.
A personal auto policy may extend to a rental
Many US personal auto policies extend some coverage to a temporary substitute vehicle, broadly on the terms that apply to the car you own. Coverage and limits mirror your own policy.
That means a policy without physical damage coverage on your own car generally does not produce it on a rental, and any deductible you carry still applies.
Extension rules differ by insurer and by state, and policies are revised, so this is a question for your own insurer before the trip rather than an assumption at the counter.
Card benefits are usually secondary and conditional
Some payment cards provide rental damage benefits when the rental is paid with that card and the counter waiver is declined. Declining is commonly a condition of the benefit applying at all.
Many such benefits are secondary, meaning they respond after any other applicable coverage. A smaller number are primary, and the difference affects whether a claim touches your own policy.
Exclusions are common for certain vehicle classes, longer rental periods and specific countries. The card's own benefit documentation is the governing text and it changes over time.
Loss of use is the charge people forget
Beyond repair cost, a rental company may bill for the income lost while the vehicle is off the road, and sometimes for diminished value and administrative fees.
Not every coverage source addresses those items, and they are a frequent point of dispute after an otherwise straightforward repair.
Deciding before you are standing at the counter
The workable approach is to establish, in advance, what your auto policy extends and what your card provides, then treat the counter offer as filling a known gap rather than as a general precaution.
Renters without a personal auto policy are in a different position entirely, since one of the three layers simply does not exist for them.