The relations between an advertising agency and a client can go on for years, although some clients do move from agency to agency. Firms such as DuPont, Procter & Gamble, Kraft, Kellogg, and General Mills rarely change agencies.
Clients employ advertising agencies because they believe that the agency can: (a) produce better-quality, more persuasive messages for their products; and (b) place these messages in the right media so that the message reaches the greater number of prospects. Clients who believe they can do better themselves set up their own in-house agencies. However, relatively few of them exist and these are in specialized fields such as retailing.