Depreciation is steepest immediately
A vehicle totaled a few months after purchase settles at actual cash value, which reflects that it is now a used car. The gap between that figure and the purchase price is real money, and it is largest in exactly the period when owners least expect a loss to cost them anything.
New car replacement is a different thing from gap
Gap cover pays the difference between the settlement and the loan balance, protecting the lender position. New car replacement pays for a comparable new vehicle, protecting you. A household can hold one and need the other, and the two are frequently confused.
The eligibility window is short and strict
These endorsements usually apply only within a stated age and mileage, often the first year or two. Adding one later is generally not possible, so it is a decision made at purchase rather than at renewal.