Its all about using the famous person to help you remember the company that is advertising. Let me demonstrate in the insurance market. These are all the companies i can think of off the top of my head (without searching):
Esurance - Jim Halpert (don't even remember the actor's name)
Geico - that lizard that makes barely humorous comments
Nationwide - Payton Manning
Allstate - that black guy who was US President in 24
The only other one i can think of is Farmers which is what my parents have.
When i shop for my own insurance in the future, i will likely choose one of the ones i remembered from a commercial simply because those are the ones I'm aware of.