Because it's how we communicate with each other and it's how you express yourself. It's one way in which you expose yourself to the world. It's one way in which you make an impression of yourself on the people you encounter.
If you are articulate (in written and spoken language) you are perceived to be intelligent, educated, and capable in your endeavors. These perceptions lead to your ability to influence others, gain respect and gain some sort of status that is elevated above the inarticulate.