I could give you a myriad of reasons explaining why you should advertise on radio.   However, for purposes of time and page length, here are some of the more important reasons:

• To position your company, services, and products.
• To create, enhance or maintain image, prestige and leadership.
• To direct "word-of-mouth" in your marketplace.
• To reposition the competition, their services, or their products
• To sell off excess inventory.
• To stand above, apart from, or keep up with your competition.
• To initiate, stimulate or maintain momentum.
• To introduce new products or services.
• To reach unknown, but powerful influences.
• To re-enforce personal sales calls, letters, direct mail, promote
  your web site, and to augment other advertising.