For 2 reasons,
1) Information about the car and previous owners can be found which could lead to the car being damaged / stolen or vandalised.
2) Plate cloning, people will check the car model, make and color and can buy a personalised plate using the car registration plate number found on "your car" and put it on a similair car in order to commit crimes, such as robbery and most common is filling up gas at the petrol station and driving of. The cameras would be played back and the car color, model and make will corespond to a differnt car such as yours and you'l get a fine.
Like if you had a blue bmw x5 with a legitamate registration plate, and another person has the same car obviously differnt number plate, he could clone your reg plate, by using "illegal registration plate" websites to create a reg plate with your reg number and letter which isnt authorised by the DSA/DVLA/DMV.