Even if a company like ours is rostered to brand manage a company, including all of the all of the graphic design work, the literature, the website, in fact all outwardly visible elements, then it is still hugely valuable to have brand guidelines in place. Even the creators of a brand can be tempted to run away with it sometimes!
Company Brand Guidelines help to 'keep a leash' on designers who, by their very nature, have creative and excited brains, often pushing ideas forward but missing some of the key rules that a brand must adhere to. Sure, you wouldn't want to stem the flow of great ideas but, at the same time, the perception of a brand is very important so shouldn't be misrepresented.
Creating brand guidelines might be a costly exercise initially but they last the life of a brand, maybe 5-10 years, or even generations, and by having guidelines you are setting a very solid starting block for the company. If brand identity guidelines are followed correctly then you can be sure that your company identity or product brand will appear stronger and more consistent over time. It is this consistency that really helps in building brand loyalty.