The Core Axis

The D string (4th string) is physically in the center of the neck. It is the bridge between the bass riffs using the low E/A strings and the melodic lines on the treble strings.

Octave Center

The D string is the easiest place to find octaves for both the Low E and A strings. 2nd fret D string = Octave of Low E. 7th fret D string = Octave of Open A. It's the connective tissue of the fretboard.