Hi.
With an agreed value car insurance, you agree what your vehicle is worth when you take out the cover. This value is set for the length of the policy which is usually 12 months.
If you didn't have agreed value, if you made a claim it would be settled on a market rate value basis at the time of the claim. Vehicles can depreciate over time, so you could end up losing money. It would be between yourself and the insurer decide what the vehicle is worth.
Regards,
Dan.