If a vehicle is damaged in an accident and then repaired, it can be argued that the vehicle is worth less than it was before the accident. This can depend on a range of factors such as the type of damage sustained, the calibre of the vehicle and the uniqueness of the vehicle.
Or think about it like this; if you had an antique vase worth $1000 for example, you dropped it and then glued it back together. The vase is still a vase, it holds water and flowers, but is it still worth $1000? This is a scenario a UK Judge used when he ruled that an accident damaged vehicle had suffered diminution.