I'm a Consultant and sometimes I help companies develop Mission, Vision and Values Statements. I'm going to go one step further than Kelly...I think each department SHOULD have its own "purpose" statement. It helps individuals within that department really connect their own work to that of the department and, ultimately, to the organization as a whole.
It's even a great exercise to have each individual within the department write a statement answering this question: Why does my job exist? It can be more difficult than it appears to answer that, but do so gets to the root of what each individual person brings in the way of "products and services" (talents and deliverables) to the table. After all, each of us is a mini-corporation (organization) in our own right and our customers, either internal or external, pay us for some outcome....not just for showing up.
I am the Party