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.
Dwight D. Eisenhower. From a speech to the National Defense Executive Reserve Conference in Washington, D.C. (November 14, 1957) ; in Public Papers of the Presidents of the United States, Dwight D. Eisenhower, 1957, National Archives and Records Service, Government Printing Office, p. 818 : ISBN 0160588510, 9780160588518
General and U.S. President Eisenhower uses this expression, learnt in its years in the military, to reveal the importance of planning ahead to prepare for the thinkable; but warning that plans will probably be abandoned in the case of the unthinkable.
The planning exercise prepares us for what we can advance. And those plans will let us go through a big bunch of common (expected) situations.
But when a true emergency arises, when the unexpected happens, the plans will not really help that much. The statement humbly assumes its incapacity to dealt with that. But forces us to not let that things that can be foreseen become emergencies.
Planning implies a reduction of uncertainty. Acting on improvisation assumes a big deal from ourselves and others; even worse given our tendency to overestimate our own capacity.