Dogs have whiskers because they can pick up slight vibrations, almost like a 6th sense. This is why dogs freak out before a storm because they can feel the vibrations from the storm in the air around them.
It has nothing to do with smell.. I agree with "ent1" where if the whiskers wont fit they won't. Unlike mammals some catfish have a ridiculous amount of taste receptors in the whiskers to find fish in the muddy brackish waters of rivers.