The practical/improvisational benefits of "hard mode" become apparent when improvising over a static harmony or vamp.
For example, consider improvising over a D minor 7 chord for 16 bars at a time. While any decent jazz musician will have
a large repository of harmonic and melodic techniques at their disposal for a single chord, the real fun often begins with
harmonic substitutions. The first and most obvious substitution is to imply the relative V7 chord, in this case, A7.
This ability to instantly know any chord's associated V7 (and its related scale-degrees) helps open the doors
to a wider and more colorful improvisational palette.