Although modes are often thought of as nothing more than scales that start on a degree other than the tonic, it’s important to understand that modes function primarily as a method of modulation. In fact, those two words are so closely related that one should never be too far from the other.
An example: Implying a Phrygian mode that begins on E while in the key of C Major has very little effect. But to insert a Phrygian mode that begins on C while in the key of C major is a fairly dramatic modulation to the key of A-flat major; four accidentals are added and the tonal center shifts a major 3rd away from the home key.
We looked at how tetrachords are formed in earlier posts. Although they consist only of four notes, altering any one of them results in a change of key. In polyphonic music, such as that of Bach, tetrachords play an important role because the harmonic material that is created is the result of the movement of independent voices.
Looking at the larger heptatonic modes which consist of a lower and upper tetrachord, there is a much clearer picture for the formation of triads and chords with higher extensions. Most modern music falls into homophonic categorization because of the clear distinction between melodic information and harmonic information.
Keep in mind that the idea of modes in this context relates to how they work in functional harmony. These concepts do not apply to what might be considered modal music, a term that could be applied to a variety of composers, styles, genres, and time periods.
The distinguishing factor that underlies modal music is that it removes the idea of modes serving any traditional harmonic function; the idea of Tonic-Dominant relationships do not necessarily serve a purpose, although one can never be too sure. It could be said that composers and improvisors of this ilk are likely drawn to modal music because of having a deep understanding of their intended functionality.
Also, there’s always exceptions.