Where does the advantage lie in space, with the offense or the defense? Why does it matter? Put simply, perceptions of the balance between offense and defense determine doctrine. Doctrine, in turn, drives acquisition, and what military space operators acquire further reinforces the existing perception of the offense-defense balance.
As a result, a nation's military is equipped and prepared to fight based on its perception of whether the offense is stronger or the defense, not the reality. It's only when war starts that a nation's doctrine and equipment are tested against the reality of its perceptions. The wrong equipment and doctrine often determine the winner and loser in any conflict (see France in 1940). So accurately assessing the strength of the offense and defense before conflict even begins is one of the fundamental determiners of success in war.