Because the foundation of your business are your buyers! The purpose of this exercise is to know who they are, what they are thinking and what they are feeling. Both when working with you and outside of your business relationship.
Personas are such an important part of every marketing strategy and plan. After you have your vision, mission, and brand pillars, personas are the next building block. If you are looking for an awesome resource, I like this one from HubSpot.
You can have fun with them too. Make them sound like real people, because they are real people. This is the perfect chance to pull out your sticky notes, use the white board, and bring the team together. Enjoy!