Ive seen all kinds of brand guides, from hilariously terrible to awe-inspiring. When done well, a brand guide helps to create consistency in internal and external communications and saves people tons of time asking and answering questions.
In 1999, I bought a manual transmission car without ever driving one before. It was small, black, shiny, had a spoiler and I was sure I looked great in it. The salesperson showed me how to drive it in the dealership parking lot and somehow I made it home. The next day I drove it from Des Moines, IA to Chicago, IL.