functionalism

In Malinowski's famous formulation, myths are a charter for society. That is, myths shape and reaffirm a culture's social structures. For example, the complex social relations among the Greek gods confirms the "rightness" of the Greeks' complex social structure; conversely, the largely undifferentiated social structure of the Germanic gods reaffirms the "rightness" of the largely undifferentiated social structure of the Germanic peoples.

Myths are not necessarily a mirror of society. Incest among the Greek gods does not mean the ancient Greeks regularly practiced incest. Instead, it reaffirms the hierarchical relationship between gods and humans -- gods are not constrained by human social relationships.

Myths, of course, have other functions than as charters for society. In general, though, myths tend to be conservative, reaffirming the rightness of the status quo.