Why do you need a creative brief?
Assume you've already decided on the contractors who will create your commercial's materials. You can specify the types of outcomes you want to achieve and what you want to tell your target audiences based on your brand model.
You must properly introduce the project to these contractors in order for your work with them to go smoothly. You must phrase the goal, create a proper document, and translate everything from business language to the creative team's working language.
The creative brief is the name of this document. I'm able to set the right goals and formulate a creative brief because I've been in the creative industry for 18 years as both a creative unit and a manager.