The settlement is the value, not the loan
A total loss pays actual cash value: what that vehicle, that age, that mileage, was worth the day before the crash. A newer car financed over a long term can be worth less than the outstanding balance for much of the loan, and the difference is still owed on a car you no longer have.
Gap cover closes exactly that difference
It is inexpensive, it is sometimes already included in the finance agreement, and it is sometimes sold twice to the same person. Knowing whether you have it, and from whom, is a five minute check that is worth doing once rather than never.
Depreciation moves faster than the payment schedule
The steepest drop in value happens early, while the balance is still near its highest. That is the window where the shortfall is widest and where people are least likely to have thought about it.