Building a brand identity guideline is essentially a set of rules that you can use to explain how your brand looks and projects itself.
The brand guideline will cover fonts, colors, imagery and image treatments as well as advertising and graphic language. When you set out to build a brand it is important to communicate with consistency across all platforms from a messaging as well as visual point of view.
Building consistency will strengthen your credibility and brand image in the public domain. As the brand grows there is a chance you may have to hire third party suppliers (eg; advertisers, graphic designers, photographers, printers, etc.) the brand guideline will ensure that they can easily get a picture of the brand and fall in-line when creating new communication elements and that they are synecdochic with the rest of the brand.