One answer to that question can be found in the introduction to Air Force Manual 1-1, Volume I, March 1992----

"Aerospace doctrine is, simply defined, what we hold true about aerospace power and the best way to do the job in the Air Force. It is based on experience, our own and that of others. Doctrine is what we have learned about aerospace power and its application since the dawn of powered flight. While history does not provide specific formulas that can be applied without modification to present and future situations, it does provide the broad conceptual basis for our understanding of war, human nature, and aerospace power. Thus doctrine is a guide for the exercise of professional judgment rather than a set of rules to be followed blindly. It is the starting point for solving contemporary problems.

Doctrine is also a standard against which to measure our efforts. It describes our understanding of the best way to do the job---the world as it should be. Many factors can prevent us from acting in the best manner, but doctrine can guide our efforts, gauge our success, and illuminate our problems.

Doctrine should be alive---growing, evolving, and maturing. New experiences, reinterpretations of former experiences, advances in technology, changes in threats, and cultural change can all require alterations to parts of our doctrine even as other parts remain constant. If we allow our thinking about aerospace power to stagnate, our doctrine can become dogma.

This is an airman's doctrine---written by air power scholars for use by air power practitoners."

And there you have the essence of what doctrine can and should be. Tactics, techniques and procedures provide the details.

Why can't all the Services have an upper tier of doctrine manuals (brief not limited to an arbitrary 200 pages, and replete with quotes from leaders past and present, and historical vignettes to provide context for the application of doctrinal principles), and a lower tier of tactics, techniques, and procedures manuals?