Plans are nothing, Planning is Everything- General Dwight D. Eisenhower
While Eisenhower didn't say this quote as an original. That was "No Battle Plan Survives Contact With the Enemy" - General Helmuth von Moltke.
The quote is used - and misused - by most agilest and in the Agile Manifesto. Of course those authors probably didn't have access to the original context. Eisenhower was speaking to the National Defense Executive Reserve Conference in Washington D.C. on November 14, 1957. The quote in full context reads:
I tell this story to illustrate the truth of the statement I heard long ago in the Army: Plans are worthless, but planning is everything. There is a very great distinction because when you are planning for an emergency you must start with this one thing: the very definition of “emergency” is that it is unexpected, therefore it is not going to happen the way you are planning.
We know this for sure - any forecast about the future is wrong. All forecasts are predictions about the future, so the forecast has to be wrong. Is the forecast worthless? Of Course not. But all Plans and Forecasts must have the ability to adapt to the emerging situation
The plan establishes a goal and provides an outline and a starting point for allocating resources, delivering outcomes in the needed order, determining conflicts and dependencies. Anyone making plans and forecasting the outcome from those Plans must understand the limitations of making decisions in the presence of uncertainty.
When the conditions change - as they must in the presence of uncertainty - who best can respond quickly and make adjustment the plan or forecast? Of course the people who built the plan!
Planning is Strategy Making. All strategies are hypotheses. Hypotheses must be tested with experiments. Experiments provide actual data to adjust the Hypothesis, which in turn adjusts the Strategies, represented in the plan.
So when we hear from the Agilest who quote the Agile Manifesto - Responding to Change over Following the Plan to mean we don't need a plan, because we're not going it anyway. They have made a fatal error in understanding.
To respond to change - as the manifesto says - means we have to have a Plan to know what we are responding it and how our responses will impact to goal of the project - which the user has stated (or should have stated as the Needed Capabilities). Otherwise everything is a change and the Product Backlog is just a pile of ideas - not a map of anticipated value to be delivered to the customer.
And of course - since the future is always uncertain, when we encounter the naturally occurring changes and the event based changes, we'll need to estimate to impact of those change on our Plan to meet the Goals for the project. And we'll need to estimate the impacts of the alternative choices to get back on the path toward our Goal.
Without making those estimates we're in an Open Loop Control System being whipped around by externalities , creating extra work, building technical debt, and increasing Work In Progress, and generally mismanaging the project.