Because you don’t. A topic in a couple places of Michael Bierut’s presentation last week was a challenge to the notion that designers always know best. They know the best type, the best visual, and the best overall solution to the problem. This can be the case, but not always. Michael Bierut has experienced this. I’ve experienced this. Many times. The client or another collaborator brings the solution. It’s not uncommon. And you, the designer, need to be listening enough to recognize and grab hold as needed.