Without a written plan, people can more easily claim plausible deniability when things go south. On the other hand, if you write down your assumptions, and map out how you're going to achieve your goals, you can at least plan to meet them.
The Rest of the Story...
What we often find so funny is the number of cases where companies have these elaborate KPIs they want us to hit, and they have absolutely no clue as to how to hit them.
They just assume it can be done because, because?
So then, when we tell them that we need to write a plan, they're like, why?
OK. Ready, fire, aim.