In Onward: How Starbucks Fought for Its Life without Losing Its Soul, the CEO explains that they once strayed from their mission. He also explains how he views the relationship of mission and purpose:
Starbucks’ mission statement had never been just some framed piece of paper posted on our offices’ walls. Perhaps more than any other company, we had for years used our mission as a touchstone to make sure the guiding principles of how we run our business are intact and as a measuring stick for whether or not the company is aligned with its founding purpose, which as the highest level is to inspire and nurture the human spirit.
The mission is a tool; it is a measuring stick for whether or not the company is aligned with its founding purpose.