"Language is a system of differences without positive terms." This is the first salvo in the closing off of the operation of language from reference--meaning. It almost works. Grammar requires that each word in a sentence have a function--a function in that sentence. It does require that any part of that sentence have a meaning, a referent, real or imaginary. It does not require that the sentence have any intent or function either of expression or of communication.
This matters, it seems to me, a lot and a little. A lot because language can be divorced absolutely from any of its assumed functions--conceived as as proper or ancillary; little because nonetheless this substance of neutral, naked, empty signifiers can be used for all of the functions for which it is used. We can, it seems to me, communicate and express by means of language most effectively. We can do this things outside of language, so called, as well--often better. The question must be turned around: it is not "what is language for? What does language do?" But rather "what do we do with language?" "What do we use it for?"
What becomes possible because we use it? (What do we use it for and nothing else?) What would be possible without it? What would not be possible without it? (Meaning could be created and conveyed, but how precisely?) Where are its borders? Where are its centers?
Why is all meaning metaphored in notions of carrying: metaphor, convey, communicate? But that's another matter.