A standard policy pays the actual cash value of a car, which can be far below what a classic is worth. Collector policies use an agreed value set when the policy starts, so you know what you will be paid after a total loss.
In exchange, collector insurers usually limit how you use the car: pleasure driving and shows rather than commuting, secure storage, and another vehicle for daily use. Premiums are often lower than a standard policy because these cars are driven less and cared for.