By Defining your brand, you'll be able to be more organized in your thinking about your marketing and designs.
Instead of designing things on a piece-by-piece basis, you'll have your brand communication goals recorded and available for you – and for your designers, writers and other marketing team members – to consult. Or, if you're a very small business, you'll be able to consult this definition over time.
This way, even if you have multiple people creating your brand, they'll all be working from the same goals, and trying to communicate the same things. This, combined with creating a record of the words and images you use to communicate your brand definition (i.e., do you know what phrases you use throughout your materials? How about having copies of your logo and stock photos?), can help you to create a consistent and well organized marketing effort moving forward.