The purpose of a brand guidelines is consistency.
“Many parties will be touching the brand—both internal and external—so it’s very important to define a set of rules for all of them. Customers are more savvy these days, and expect a brand to show up in a cohesive manner. A brand lives on mobile screens, in social, on the web, in the streets, in TV and film, and even the traditional business card. All touch-points matter.” This is a go-to survey article.
Article: How do You Make Brand Guidelines?