Humans are not created equal, political ideals notwithstanding. Visionary, practical, dutiful, impulsive, sensitive, brusque, intuitive, realistic, creative ... all sorts occur among us, and psychologists have labored to classify the most important and immovable types of psychological variation. Carl Jung came up with one much-used four-dimension system, nicely described in a book by Keirsey and Bates: Please understand me. (New edition.) One of the most salient of these dimensions is introvert/extravert. As Keirsey and Bates relate, extraverts give parties, and leave from them energized. Introverts leave drained, and seek private spaces and solitude instead. Extraverts feel lonely when they are alone, whereas introverts feel alone when they are in crowds!
The population apparently divides 75% extravert to 25% introvert. Naturally, extraverts run everything in society, from political affairs to social clubs and businesses. They also think that introversion is a problem- that "social skills" are essential for a "normal" life, and that not wanting to participate in social affairs is excentric, deviant and disreputable. One has to ask how introversion ever arose, and what keeps it at a stable proportion in the population (it is strongly heritable). One approach lies in evolutionary psychology- asking about the settings in which we evolved. Currently, we live in huge societies where introversion can be quite painful, and it is common for many to want to get away from the crowds into their own single-family home, cacooning in their own space with their own home theater system, etc. Clearly we never evolved at the population densities of today, but in far more modest tribal or extended family settings, where introversion was less of a problem.
So how is (or was) introversion beneficial? Social life is far from the whole point of human existence. Though being "inner-directed" may sound egotistical, it is actually quite different. The introvert's focus on and sensitivity to non-social issues may yield better perceptions of reality- better vigilence over the natural surroundings, for instance, which can benefit everyone. Art also has little to do with social issues, but is an exploration of the inner life- what it means to be human, what kinds of novel views one can take of reality and of ourselves, what heights of feeling humans can attain. And then there is spirituality, the ultimate inner experience. When it comes to deep feelings and spiritual impulses, introverts are more likely to have them, to pay attention to them, and protect them from the mundane intrusions of the external world. That is, to be inspired. As the joke has it, spirituality is what introverts do to make extraverts feel guilty.
Thus the constant seeking that many people experience for traditions and gurus who have the most deep and esoteric things to say about the human condition. This goes back to the position of the shaman, an apparently constant part of early human society, though present in an infinite variety of incarnations. These explorers of the inner life are respected for their special powers, sometimes amplified by vigorous PR and the incomprehension that extraverts have of this other mode of life. Some of these powers are undoubtedly real, since shamans cultivate alternate views of reality that can be important antidotes to the constricted, reality-driven views of the majority. Keeping alive a sensitive, inspired, and imaginative approach to life has great utility, and human populations doubtless benefit from a vibrant mix of the various personality types- a mix that is apparent in the work of Jane Goodall and others on chimpanzees as well.
Ironically, once societies grow in size, their spiritual/religious functions are captured by those who run everything else in society- extraverts (whether tied to the state or not), who have little idea where the original inspiration came from, who take the figurative ideas of the prophets with unimaginative literalism, who create hierarchies of "spiritual" bureaucrats, and who ultimately "stone the prophets in their turn", in the words of William James. However, when the religious marketplace is free there is a constant turning to new prophets, sects, and movements that can put people in touch with the wellsprings of spirituality without the accretions of extraverted (and spiritually dead) organizations, ceremonies, and dogma.
Incidental link: an illuminating analysis of the political campaign last spring in personality terms, involving, naturally, extraverts exclusively.