According to Merriam-Webster dictionary, language is:
‘a systematic means of communicating ideas or feelings by the use of conventionalized signs, sounds, gestures, or marks having understood meanings’.
There is no doubt that the most common use of the language is the communicative. We need the language whether to have a intimate conversation with our friends, or give a speech in front of hundreds of people, but the main purpose is to transmit an idea, a thought. To communicate.
But is it the only one? Certainly not. We use the language for other purposes too.
Imagine you are in a bar in, let’s say in Tokyo, and you spend an evening talking to your friends. You’re not ‘communicating’. You’re rarely communicating. You’re not presenting them with any information that changes their belief systems. You’re simply engaged in a kind of social play. You’re establishing social relations and creating warm interactions or determining your relationship to someone or whatever. This is the social function. I agree that this could be considered derivative of communication, but communicate and socialize are not always the same thing.
In the same way, you’re not communicating when you are having an argument with someone. You’re not listening what the other person has to say. You’re arguing. You’re just expressing your opinions or your emotions. There is not an exchange of information there, so there is not communication neither.
Finally, the language has a cognitive function, which means that humans, we use the language to transfer our thoughts in and out of our mind. Have you ever realized that when you think, you do it in a particular language? In my case, I do it in Spanish, but it happened to me that sometimes, I do it in English as well. It is very common when we are living in a country where our mother language is not spoken, we end up thinking in the new language. So, we use a language in our inner speeches, and we are not communicating at all.
Language is the blood of the soul into which thoughts run and out of which they grow.
~Oliver Wendell Holmes