When a vehicle is temporarily imported, the host country will issue to it a temporary registration number. Where the car's own number is readable in both countries (i.e. not Arabic letters etc.) the temporary number will be the same and the car's own plate will be used. The GB sticker (which only costs a few pence anyway, or is sometimes given away free by travel companies) is to help the authorities in the case of an emergency or traffic offence. Your question "why" is valid, however - who can read the tiny letters from an EU plate when the vehicle is moving?