N.B. The following material © Alan Belkin, the giving full credit to the author. Although charge, provided the authorship is clearly This book is the first in a series of four. Why this book? Why this book?
The point here is that virtually all harmony books assume complete and continuous unity of tone. In real life, however, there is a world of expressive potential to be explored in multi- layered harmonic textures, and in various degrees of blend between planes of tone.
The basic principles of voice leading arise from two fundamental facts (for experimental documentation, see Albert Bergman's Auditory Scene Analysis) : the tendency of the ear to separate musical strands by register, and the fact that voices (and most instruments) are most at ease moving by fairly small intervals.
All other things being equal, the more complex the harmonic world, the harder it is to sense non-chord tones. While there is no absolute reason for using non-harmonic tones, they: allow for more subtle gradation and variety of harmonic effect, lighten the texture, and allow the composer to follow his melodic impulses more easily.