The moment that you think the main purpose of your group is so well established and accepted that to emphasize it is to restate the obvious is a dangerous moment indeed.
Your organization's mission must be reviewed frequently. People forget. Turf wars are easily justified. Details overwhelm the central reason for the organization's life. Individuals and teams wander from the main trail and into the jungle.
The mission must be the mantra. All should know and employ it as a benchmark when determining the propriety of actions so time is not squandered on side matters.
To make it memorable, it should be brief and easily recalled. Far too many mission statements are lengthy and arcane. They are quickly posted in the conference room and just as quickly forgotten. Many are mocked by employees who see the difference between what is proclaimed and what is practiced.
That suggests another benefit of frequent reviews of the mission: To root out hypocrisy.