That's why companies like this type of sponsorship or having brand ambassadors. It's because the fan base of these celebrities is also becoming their potential market so, for them, it's the true power of marketing that the others are also going to benefit from hiring or signing them with a contract. But this is one of the most expensive ways of advertising.
Indeed but in terms of effectiveness especially if the promoters is a well-known personality the said target venue can be penetrated and earn interest even from those who doesn't know or who doesn't need whatever you are offering, they just gain that interest because of the person who's promoting the product.
Expensive but there's studies that can identify the potential and in terms of business there's always a risk that's needed to take and needed to consider.