Carolyn:
I have often thought this would be a great topic for Champs. You should present this.
I did a talk on this same topic for Rookies at a local kick-off years ago.
Just a few practical benefits that we have found:
1- Focuses the team on the messages of FIRST
2- It forces the team to reflect on and evaluate their past, present, and future activities
3- It gives the team a document that they can share with potential sponsors and supporters.
4- It creates a team history and is an informal way to track the development of your team.
5- It creates an excuse to educate your entire team about team's mission
Just a few of the many benefits I can think of on the spot.