Freedom is one of the greatest restraints that you can impose on a graphic designer when trying to define your brand. Guidelines are a must. Consistency is crucial. You don't want any information to be lost when growing or developing your brand, and a style guide helps you maintain a steady image.
Choices on colors, fonts, and imagery become much more daunting when there's an entire universe of options. Big brands have style guides that keep their communication efforts connected. What colors are allowed? How should the logo be used?
The less time a designer has to spend figuring out which elements need to be used, the more efficiently they can put out a good product. If the designer knows they have to use Proxima Nova to keep brand consistency, the less time they have to spend thumbing through the other 1,257 fonts on their hard drive and the more time they can spend using Proxima Nova well.
Does your logo or branding consist of multiple colors? Know what colors to use when is an integral piece of every design puzzle.
Let's say you use highly contrasting colors like navy and orange, for example. These colors go well together. A vivid orange is much more vibrant than a navy blue, so it might be wise to use the navy for backgrounds or text and the orange for buttons and to highlight essential information.
Imagery is an important factor as well. What kind of images should be used? You need to figure out what kind of visuals resonate with your target audience. This can be loosely defined or highly specific depending on your brand and audience. Further things to consider would the style of the imagery. Does it work for your brand to always utilize specific kinds of images? Or to always crop them to squares or circles? If you start implementing a specific process, you should aim to keep that consistent.
Criteria like typography, color scheme and imagery can help control your brand. This is important because when you have control of the brand, you have control of the message. So whether your style guide is a simple one page document or something a little more intimate, it's an important tool to have for your brand.