The aim of linguistics is to go beyond the study of individual languages to determine what the universal properties of language are, and to establish… - David Crystal

The question “Why do we use language?” seems hardly to require an answer. But, as is often the way with linguistic questions, our everyday familiarity with speech and writing can make it difficult to appreciate the complexity of the skills, we have learned. This is particularly so when we try to define the range of functions to which language can be put.

There is little scientific data on the point, but evidently people do speak to themselves.

The structural properties of the language are many and complex, but at least they are finite and fairly easy to identify: there are only so many sounds, letters, and grammatical constructions, and although there is a huge vocabulary, at least the units are determinate and manageable.