One of the broadest trends in contemporary style is microtonality, or using more than twelve distinct notes per octave. While such a definition appears quite simple, the applications of and intentions behind "microtonality" can diverge widely, both in their artistic and historical motivation. Examining the meaning & context of "microtonality" will clarify the (at least) two separate intents behind the movement, as well as provide some insight into the development of musical conventions. To begin, the structure of the word "microtonality" subsumes a Western concept of tonality, and this is something to be set aside immediately. While one cannot entirely separate functional characteristics such as key relationships from the development of scalar-tuning structures, it is also clear that the coalescence of Western tuning theory into twelve-tone equal temperament was not particularly motivated by harmonic relationships. If anything, it was done in spite of them. The complexity (or lack thereof) of functional relationships was accommodated & optimized by distinguishing only twelve notes per octave, and one motivation for microtonality has been a desire for increased complexity and hence increased distinguishability.
Microtonality is about distinguishing notes more than it is about adding to sonic density. Even the most straightforward historical tunings distinguished between sharps & flats in ways which twelve-tone equal temperament does not. Moreover, even while twelve-tone equal temperament was officially the tuning of Western music, musicians capable of accommodating a finer degree of tuning (especially the string quartet) did so. In that sense, there has long been microtonality, as the precise pitch of notes which were nominally the same was adjusted to fit the context. From that perspective, much of explicit "microtonality" has been about specifically realizing a diversity which standard notation already partially captured. Such concerns were already prominent in the sixteenth century, in proposals for keyboards with split keys, and other systems to accommodate modulations perfectly in tune. Gesualdo's harmonic relationships, for instance, are now seen not so much as dissonant, but as taking great care to connect modulations with precise "microtonal" tuning. In effect, the microtonality makes the music seem less dissonant, and it is on this point that I want to divide the two fairly distinct uses of the term.
The entire idea of "dissonance" is a thorny one, built as much from context as from any hard physical facts. However, if tuning a note more distinctively serves to clarify its relationship to other notes, one can suggest with some degree of precision that microtonality has served to make it "less dissonant." That such modifications and accommodations should be undertaken by the performers has been a given. It was equally true before Western notation fully distinguished sharps & flats from unaltered notes in the form of musica ficta, and continued to be true on a smaller scale as pitch notation was refined. In the case of notation to specify tuning relationships, the impetus is a desire for more control on the part of the composer. This is true for any refinement to notation, and in the case of microtonality, it explicitly demands intonational refinements which may or may not have occurred otherwise. The most distinctive cases are, of course, the latter, and this additional control has given composers the ability to play one tuning system off another in the same piece, or to exploit motivic relationships which would have essentially dissolved into the relative imprecision of standard tuning. The discovery & use of historical tuning systems — as well as world tuning systems — has provided a fine impetus for such explorations & juxtapositions, and indeed any conservative backlash suggesting that the audience is physically incapable of hearing these distinctions is refuted in practice by such systems as Indian classical music. There, a recognition of the finer points of tuning is sometimes necessary to tell one raga from another.
Note, of course, that Hindustani music accomplishes these distinctions with a theory no more involved than ours, i.e. sharps & flats. Further distinctions tend to be described imprecisely, and conveyed orally. The inclination to settle on more explicit prescription is there, however, as it is in Western music. Especially as more styles coexist closely, the idea that a composer should — at the very least — specify a basic tuning for his piece becomes far from frivolous. With technological advances, more instruments can accommodate such variety, and so any motivation for using twelve-tone equal temperament becomes as much historical as practical. The counter-balance to enthusiasm for more precision in tuning (or what some call "just intonation") is, unfortunately, the industrial degradation of human hearing. Demand for something as basic as intonational precision can easily be seen as elitist too, fundamental complexity aside. Such impressions can be readily observed in basic listener reactions to manifestos on microtonality, namely that they imply a greater degree of dissonance. Much of the historical motivation behind "microtonality" is just the opposite, but the association of sonic complexity with dissonance is strong. Musical construction which demands that the ear distinguish more than twelve notes obviously places a greater burden on the listener, but the ultimate result of failing to perceive such differences is not that the piece sounds harsher, but that it sounds simpler. Its distinctions are obscured.
Microtonality used other than to clarify relationships between notes might better be termed "infra-chromaticism," and it is in this area that greater dissonance is often sought. The impetus there is to obliterate the discrete character of notes, making a continuum of sound, and then a clash of intervals smaller than a semitone. Here there is little historical precedent, although the noises of the industrial age do offer various suggestive analogies. Obliterating note boundaries can even be seen as a route to transcendence, although certainly a daring one. The "anything goes" pallet of infra-chromatic can easily bewilder the listener, even as it offers something of a natural extension to tuning refinement. Although the notation required is essentially similar, infra-chromaticism is more the limit of twelve-tone equal temperament than it is related to the quest for tuning precision driving the first type of microtonality. Microtonality derived from juxtaposing various tuning systems should eventually have a similar limit, but for now its exploration is much more cautious, as much to separate it from the disorientation of infra-chromaticism as for any other reason. The "whys" of using more notes can be very different indeed.
Behind microtonality is, fundamentally, an urge to refine notation. It provides a richer pallet of specification which can be put to a variety of ends, ends not necessarily having any connection to "dissonance." Moreover, microtonality need not imply additional complexity, at least from the listeners' perspective, as specifying a particular tuning does not necessarily mean that a greater variety of notes will be used, only that those used will be used more precisely. The consequent refinement & recognition of the variety inherent to tuning also provides a structural basis for fusion, as a way to find correspondences on the most minute level and elaborate them into larger forms. Perhaps most fundamentally, microtonality reflects a way of hearing music which does not seek to regard similar notes as the same, but rather glories in their differences.