How does a celebrity endorsement bring credibility to a product

Answer: Its proof that the product works in some ways. People trust the celebrity and will try the product if the celebrity supports it. Also, people want to live like celebrities, and the products they endorse are like being able to be a little bit like them.

Celebrities are very famous people which many people look up to. It's simple marketing. If a famous celebrity says that x product is working well for them, people will believe it because they look up to that person. Then, more and more people will begin to by the product.