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Does a celebrity provide any real clout to a product? Just because you see Joe Celebrity using a product in a commercial, all that really means is that they paid her enough money to say that she uses the product. There have certainly been celebrities outed for not using their endorsed product - for example, when Samsung paid a number of celebrities to endorse their phone, then the celebrities were caught tweeting or…
As one example, Nespresso hired George Clooney as the face of its advertisements after years of him being featured in women’s media as one of the sexiest men around. As sex drives so much of human behavior, do you really think that Clooney’s presence on this brand’s advertisements had no effect on at least some consumers?
He certainly did, and that's the precise reason why they hired him instead of say some coffee expert that would cost them 1/100. They don't even need celebrities; just look at how many hot girls you see at motor shows posing by the various cars: they're 100% unknown but the induced association "You buy this car -> You'll get laid with her" will help sales for sure.