WHY?

We've all seen these numerous times, we find them fitted to virtually any electrical appliance plug we buy, yet we haven't a clue why they use them? If it's to protect the plug pins from oxidising... maybe? I doubt it...since every electrical store selling just the 3 pin 13 amp plugs, offers them with no plastic shrouds...so why? Looks rather as though some enterprising plastic manufacturer has sat back for years and coined it in, after persuading the manufacturers of appliances to use the things? The landfill sites must be full of the darned things :(