It gives the underwriter some idea as to the condition of the car - and if you say it is worth loads more than the normal there would be a reason to ask why e.g. modifications etc. SOme insurers have a limit on value and they will not accept cars worth more than that limit.
Risk assesment. We ask the value of the car for a couple of reasons. If the vehicle is valued over £5k, we make it compulosry to have comprehensive cover. We are obliged to ensure that a car has what is deemed as 'reasoable cover'.
Anything over £30k & we will require a tracking device to be fotted to the car. The higher value the car, the more desireable, the greater the risk.