They need them to create a need in people to go out and buy something they probably don't really need. And the techniques the commercial actor uses to accomplish this dubious task are offered in schools around the country. I studied commercial acting at the Weist-Barron School of Television in New York City. Just take the bottom of your top teeth and place it on the top of your bottom lip. When I watch commercials, and I see the actors doing that, I know the difference, but it doesn't really matter.
Nobody pays that much attention.
The actor got a nice residual check, the sponsor sold a softer toilet paper, and the consumer treated himself to a properly pampered posterior. Then there are the heavyweights. The undisputed kings. The dramatic actors who etch their performances into the center of your soul, and leave a permanent mark there. Al Pacino's done it.