A good mission statement is an aspirational goal that helps everyone move in roughly the correct direction.
It is not strictly descriptive of what the organization is currently like, but it should reflect the best moments attained. An aspiration that is not grounded in the possible is easily ignored, or worse yet leads to folly.
It should be short. “The bandwidth for communication in a large organization is about six words”, says @clintsharp, meaning you can’t get complex ideas from top to bottom or side to side. Try for a fifth grade reading level, not a doctoral dissertation. Simple words, simple sentences, declarative voice. Picking on a semi-random example, AT&T: “to exploit technical innovations for the benefit of AT&T and its customers by implementing next-generation technologies and network advancements in AT&T’s services and operations.” That might seem a little mercenary for some folks, so it’s softened with this statement of values: “Live true. Think big. Pursue excellence. Be there. Stand for equality. Make a difference.” Note the values statement tries to follow the same rules, but has to have a 244 word explainer page with lots of pictures and shouty fonts. “Exploit technical innovations for the benefit of AT&T and its customers” doesn’t need a lot of explaining, but if you’re still not clear on your job in the biggest American telecom, it’s to “implement next-generation technologies and network advancements”.
If it is working correctly, the mission statement is used in internal disagreements and may help to settle them. Reasonable people will disagree about implementation details, but if a proposal is clearly not in line with the simply stated mission, it should be rejected. Organizational failure to do so doesn’t mean the mission statement is bad though. Google’s mission is to organize the world’s information and make it universally accessible and useful. Seems clear. So how the heck did this happen? Maybe the increased focus of an enterprise software instead of consumer and enterprise focus would help. Elastic helps people do great things with data. Splunk makes data accessible, usable, and valuable to everyone. Snowflake enables every organization to be data-driven. No chat clients yet, but maybe Zawinski’s Law is still coming for them.
If it is not working correctly, the mission statement may be ignored or mocked as a pointless artifact. This can happen when the aspiration is too vague or too disconnected from day to day reality. If your mission statement says to do a thing that no one in the company ever works on, it might as well say that you’re all here to solve the problem of warp drive.