It helps stop vehicle cloning.
I can walk down my high street and get a Ford Focus reg number. If I used it to clone a car and drive about in the same area, it wouldn't be long before it was spotted.
Whereas, if you post a pic of a Ford Focus with number intact, I can use that number to clone the car and as you are in Isle of Wight the chances of the two cars meeting are very slim.
All that info and never even left the house, the joys of t'internet.