Think about a book. Whether printed or braille, the page groups characters into words, words into sentences, sentences into paragraphs, and paragraphs into chapters.
Consequently, it is impossible to confuse one with the other, and the more you read, the easier it is to distinguish between chapters, paragraphs, quotes, and dialog.
Branding is based upon the same principles. Like a page, a design consistently organizes logos, images, and content. Unlike a book, designs are not grouped into chapters. Instead, they are spread across different media like print, web, or broadcast. As a result, people may never see your full book.
To create consistency, a design's basics are repeated. With each exposure, an impression of a company is created and reinforced, just as you get an impression of an author and his book with each turn of the page.
Imagine that same book again. Only now every paragraph looks different from the one before it - using different fonts, sizes, margins, or even styles.
Not only would that approach interfere with the legibility of the book, it would add to the cost. Each paragraph would have to be formatted differently, increasing both the design and approval times.
Branding is no different. Where consistency is the goal, there is a financial benefit.