Communication is vitally important to all parts of business, from internal memos to press conferences and releases. Without open and coherent communication, a business can become obscure in its vision and dishonest in its practices.
In marketing, communication plays a specific role. A product or service must be promoted to the public so they can compare and contrast it with competitors. Advertising, media appearances, and branding are all important parts of marketing communication. Well-written ads ensure that the public is interested, and branding ensures that mental association is made for the brand rather than the product itself -- this is why many "generics" are available but the more expensive and identical "brand-names" remain popular.
Media appearances are important because they allow the product to be seen in a formal and theoretically objective environment. This is a form of "vetting," and allows the promoter to showcase the unique or useful aspects of the product. Modern infomercials are a guided form, while live commercials on shopping networks are subject to mishap.
Communication also has a role in damage control. "Spinning" the report to be more favorable is a common business practice and there are whole industries devoted to it. If a product is found to be unsafe, or indicated in accidents, marketing needs to communicate with the media and public to reassure that the product itself is not to blame. This may or may not work depending on the situation.