Sun Tzu said (roughly):
If you defeat his plan you defeat your enemy.
If the enemy isn't sure what your plan is, that makes his efforts more difficult.
If you cover your plan with a smokescreen of noise, or dust, it also makes his efforts more difficult.
Then again, if you don't know your own plan, you may defeat yourself.
And lastly: Ike has an infamous aphorism -
"Plans are worthless, but planning is everything".
(Pretty much every staff officer has had this as part of their syllabus).
This means that while rigid, written plans often fail when reality sets in (like in battle or crisis), the process of planning is indispensable because it builds adaptability, knowledge, and readiness for unexpected.