^ There's a lot about human nature that doesn't make sense, but in the end people base their decisions on all types of irrelevant factors and nobody understands this better than the advertising agencies. They're not paying celebrities millions to endorse their products for nothing. Do people really believe George Foreman is an expert on indoor grilling? I hope not.
It almost works in the same way as Pavlov's dogs. We associate certain people with certain qualities, and when said person endorses a product or service, their qualities lend themselves to what they're endorsing. You get OJ Simpson to be in your commercial for health insurance, you immediately don't trust the company. You get Morgan Freeman or Andy Griffith to endorse the same company, and of the sudden you feel positively about it.
So if people will buy a certain kind of underwear because Michael Jordan says to, then it's no mystery why people will buy a racket simply because Federer uses it. Michael Jordan is no expert on men's brief's, but most would think Federer knows about tennis. Because when you think of Federer you think of grace, power, and control, and so you project all those qualities to the racket he's holding, the racket being a figurative extension of his hand, of Roger himself. And on an even more basic level, if that racket is being used by the world's best player, than it must be the best racket out there. Same thing with Tiger's golf balls.
Yes, I'm freakin' bored.