While not present in these examples, the inversions of the quartal triad (a stack of three perfect fourths) often go unnoticed and are, in fact, essential to utilizing common tones in quartal harmony.
Consider the chord (from lowest to highest) A-D-G. If we consider this to be the root position of the quartal triad, then its first inversion is D-G-A (known colloquially as a sus4 chord), and its second inversion is G-A-D (known colloquially as a sus2 chord).
Why is this important? Well, consider how we use common tones in traditional tonal chord progressions:
Take a V-I cadence of G resolving to C. We would typically resolve G-B-D by keeping G where it is and then resolving the other voices upwards to spell G-C-E. We don't see it often because of the old prohibitions against parallel fifths, but in a free harmonic context we could simply move G-B-D up a fourth to C-E-G. This is the type of planing we see above; if common tones are available when moving from one quartal chord to another, then the ONLY way to hold the common tone over is to invert one chord, the other, or both (The only exception being if you move up or down by one or more fourths, such as A-D-G moving up to D-G-C).
Consider this jazz piano example:
One voicing of an Am7 with an added 11th would be A-G-C-D. Note that this is an inverted quartal
stack (A-D-G-C). If we wanted to resolve to Dm7 (with another added 11th), we could plane up to
D-C-F-G (again, an inversion of the quartal stack D-G-C-F), but it would be a more comfortable
and idiomatic to voice the chord as D-F-G-C, keeping the G as a common tone.
This is all done by way of inverting quartal stacks. In the case of the guitar chords, they are planed in this manner because, as was the case with the piano example, it is more comfortable and idiomatic to play the chord progression as a series of quartal stacks. Since guitar strings are (mostly) tuned in fourths, playing two different stacks of fourths is as simple as moving up or down a fret. On the piano, large stretches of a tenth or more typically necessitate a rearrangement of the quartal chord in order to comfortably play the chord.
As for Debussy, planing is a hallmark of his style. More over, he was evoking a style of medieval music known as organum. Organum consisted of a single perfect fifth/fourth doubled over several octaves, and most textbook examples of the style are nearly identical to the chords and voicings Debussy used in measure three.
Lastly, planing of this sort is not limited to 20th century quartal chords; see the opening measures of Beethoven's Piano Sonata No. 3, mvt. 4 for an example of diatonic planing by similar motion :)
-As a side note, it is worth noting that extended minor seventh chords and major seventh chords get some of their cadential stability because they are inverted quartal stacks:
-Am7 add 9, 11, +13 = A-C-E-G-B-D-F# rearranges to: F#-B-E-A-D-G-C
-CM7 add 9, +11, 13 = C-E-G-B-D-F#-A rearranges to: F#-B-E-A-D-G-C