The range of human behaviour, absent illusions from grand narratives, derives from individual differences. Life experience and genetics interact to paint a variety of different desires in our mind which we act on, and when we reach self-awareness, the reflection (thought-on-thought) made possible allows us to have a new type of desire -- the desire to act in a certain way. This can diverge from the way we actually act for a number of reasons. Part of the human experience is the drive to consistancy -- building a value system is the application of this drive to values. Without organisation, we find that our actions are not well-enough considered to avoid damaging values we cherish. Building a value system has one purpose of making costly and difficult value weighting infrequent. In sum, the purpose of constructing a value philosophy is to permit faster and more accurate judgement of appropriate courses of actions for one's values. Note that value systems are primarily intended and considered in the light of preventing the mind of the situation from overriding one's overall values -- while more situation-obvious factors do play a role in one's values, these are not always part of the value-analysis that takes place at the time[1].
Values are a way to think about desires, and are most useful towards that end when they are understood as small entities that interplay rather than as elaborate and rare concepts. The interplay between values is where the interesting compromises should occur and are most enlightening. Different people may describe many of their singular values differently, but it should be possible, working within a foreign value framework, to discuss the engineering involved in foreign-defined values in a comprehensible way. Values by necessity conflict -- it would be unusual for many values in a value system to survive when any value in the same system is to be fully satisfied before others are considered (claims by some political groups notwithstanding). Life (and politics) has an inescapable need for compromise, both within and outside the person, both between values in constructing a value system and between people with established value systems.
Values are about what the self desires, but are not necessarily simply understood to be selfish. With the understanding of the existence of other people and care for their welfare comes the possibility of desires for society, as just one type of more abstract desire. Kant's Categorical Imperative is a statement of one philosophical understanding of this transformation. One of the challenges for humanity is the mediation of group values (present in us since before we were human) and self values (likewise ancient). Freud describes some of this struggle in Civilisation and its Discontents, and it also may be understood in another light in the field of evolutionary psychology. Without the interplay with these two broad foci of values, society as we understand it would not exist.
An understanding of values leads one to understand the various ideas on right and wrong, moral and immoral, that have been held by various societies and people throughout history. It permits lifting of an ancient purposeful ignorance -- pretensions to moral absolutism proved to be a powerful and healthy unifying factor for societies, drawing them together to speak with one voice on crucial value questions. Religion and state have both exercised this lie, for the good of society. As the state became less central and political discussion began with Democracy, differing value systems can be seen, roughly incarnated into political parties, just as differing societies and philosophies have been constructed around their own systems of values.
A powerful explanation of values leads one entirely out of classical arguments on what is truly right -- instead, two philosophically aware people are likely to deconstruct their differences into differences in value configurations between them. As value systems are not arguable, in the end aware people who hold a value system that is not massively inconsistent cannot be argued, except by use of tricks or appeals to beauty of other value configurations, into abandoning their position. As such, enlightening stalemates (as most philosophical discussions, while not leading to new positions, still provide interesting information on the way people can think about things) is the end result of most discussions between sufficiently philosophically aware people. Use of rhetoric is thus delegated as a tool to either trap those who are not philosophically aware or who are poor at debating.