I can write a clear design doc for the game now, spell out how a system is supposed to work. But the game keeps changing under me, and within weeks the doc I was proud of is describing something that no longer exists. I can't keep up with the churn, and half my docs are already lies.
More people experience this than they realize.
You can write the design docs but you can't keep them current as the game keeps changing.
If this sounds familiar, the Library can help you find the bigger picture.