The insights gained and garnered by the mind in its wanderings among basic concepts are benefits that theory can provide. Theory cannot equip the mind with formulas for solving problems, nor can it mark the narrow path on which the sole solution is supposed to lie by planting a hedge of principles on either side. But it can give the mind insight into the great mass of phenomena and of their relationships, then leave it free to rise into the higher realms of action. There the mind can use its innate talents to capacity, combining them all so as to seize what is right and true as though this were a single idea formed by their concentrated pressure - as though it were a response to the immediate challenge rather than a product of thought.
On War, Book VIII, Chapter 1
It is very much a way of thinking, a way of conceiving social reality, something of a "yardstick", but more like an interlocking "system" of concepts that form Clausewitz's General Theory of war. The perspective is to view war in terms of political relations (essentially all questions concerning power). Over time and practice this way of thought becomes automatic, it is simply the way that a Clausewitzian views war, that is in terms of the general theory. By nature retrospective, the general theory is capable of being expanded through the interaction of praxis with theory (critical analysis), or simply praxis alone (the military genius).
The general theory forms the basis for strategic theory and much of strategic thought. It is the opposite of doctrinal speculation or the "dominance of tactics" which are sadly prevalent today in strategic thought.
Sun Tzu, if he even existed, wrote about 2,500 years ago and is still read today. He provided no general theory of war, but his thought was expanded by later writers through commentary. He provides an approach to warfare (that of the Taoist sage) and sees warfare "as the greatest affair of the state".
Clausewitz wrote 200 years ago and is also still read today. He did provide a general theory of war which forms the basis of Clausewitzian strategic theory. Later writers, much like the commentators of Sun Tzu, have been able to expand on the general theory. Also a whole series of various "arts of war" covering different political epochs is possible with the general theory, as long as they do not contradict the general theory (following Wylie here).
My guess is that in 2,300 years, if there is still need for strategic theory and strategy, Clausewitz will still be read and discussed.