A marketing consultant is a very skilled professional who understand analysis, what motivates consumers and how to put strategies in place to improve business.
Some people confuse advertising agencies and marketing consultants, assuming that marketing consultants do the same thing but independent from an agency. However, the two are very different although they perform complementary functions.
If you know what message to advertise, you would hire an advertising agency to create ads. They have incredibly creative teams comprised of art directors, copywriters and graphic designers who will help deliver the message in a fresh and engaging way through a commercial or other ad campaign.