Each advertising agency specialist is an expert in his area, so in many cases he can perform his particular function better than you might be able to. The copywriter is an expert at using words to persuade a customer to take action. The art director knows what appeals to customers on a visual basis and will incorporate her artistic skills into the ad's design. The media planner knows which type of media -- such as radio, television, newspapers, online or a combination -- is right for the message you are trying to convey and will negotiate to obtain the best possible rates.