The simple fact is, people like celebrities - by definition - and the notion that celebrities shop at supermarkets just like them - and maybe they do - is a powerful incentive for the audiences of these ads. to do the same.
Advertising is all about getting people to remember the ad., associate the product, and hopefully go out and buy.
It doesn't always work - everyone is talking about the gorilla and the Phil Collins single - which has even re-charted as a direct result of this campaign - but who can recall what the advert is actually for?
Similarly, everyone thinks that Nick Park's Creature Comfort ads using vox-pops and plastecine animals were for gas - they were in fact for electricity.
All advertisers will use any and every trick to get their products noticed, remembered, and bought, and if the current flavour of the month is celebs, then let's all jump on and ride all the way to the bank - er - supermarket.