It's not just about seeing a celeb face. There's more going on that was discovered in the 80's and 90's, I think (that's when it became a big thing at least).
Built into it is the implication that the celeb likes the product. Additionally, products and branding have become a part of our personalities (IMO bad trend). So in a way you can BE like the celeb.
You will be just as cool as michael jackson if you just hold a pepsi in your hand. Sure, none of us really think this at face value, however it's implied and hinted at.