Agreed Value is where you and the insurance company agree to the settlement value of the vehicle PRIOR to the loss. If you both agree your Camaro is worth $28,000. The vehicle is stolen or totaled. You get $28,000 (less deductible)!
HOWEVER, Stated Value is an underwriting tool, and not a policy condition or coverage amount (to some extent). The insurance carrier simply uses this number to determine premium charged, versus total loss value. The carrier does have a strong argument at time of loss, if you have undervalued your vehicle to save a few bucks, and could pay a lower value than the vehicle is worth. Example - you say your truck is worth $15,000. It is totaled. Blue book, Nada, Edmunds (whatever), shows $19,300. You get a maximum of $15,000. Another reverse example - your stated value of the truck is $23,000, Blue book .... shows $19,300, you get $19,300 (less deductible).
When classic cars you own are driven only occasionally for pleasure drives, club events, special excursions, maybe 1,000 or certainly under 5,000 miles a year, not used for business, etc., it is time to enroll them in a "classic car" insurance program and get your car appraised.