Rules aren't always fun, but they're there for a reason. Imagine if Coca-Cola didn't abide by a set of standards. Something about a purple Coke bottle just seems wrong. Well, your brand is just as important because consistency has a direct correlation with information retention.
Your brand is your company's most valuable asset. It serves as the first line of communication with a client and it is referenced every time they do business with you. However, when most people think of their brand they only consider their logo. Your brand goes much further than that. It includes brand positioning, brand personality, brand promise and brand language just to name a few.
Because there are so many aspects of your brand it is important to make sure you have guidelines to ensure correct use. Brand manuals outline every aspect of your brand and allow for a single point of reference that drives all of your marketing efforts.