We name the various and sundry objects and beings that fill our lives and, well pleased by the feeling of legitimacy and reality that surrounds an object once named, we turn to ideas and feelings. We try to map the inner world of our minds only to turn around and use these lexiconical maps to order and analyze the inner world of each individual mind. If we had no language instinct, as most animals don't, we would never achieve any of these feats. Think of communication on an animal level, restrained to a very basic expression of need, fear, or anger. There but for the grace of language go we.
Language is also flexible, almost ludicrously so.
It evolves as the societies and civilizations carrying it evolve. Words are made obsolete and dropped as new words spring into being and gradually gain acceptance and widespread use. Perhaps most interesting are dialects, the varying pronunciations of a language by speakers from different regions or social groups. It can in some cases be nearly impossible to understand the speaker though one knows they are speaking the same language. It can result in a variant on the language, as in the case of Ebonics, which is now recognized as having its own grammar, etc.