What's important about language

Got a question about chapter 6, though I'm worried that it might have actually been addressed in the text and I just missed it. So keep that in mind.

I'm wondering what it is about the usage language that gives its integral role in cognition. It makes sense that the thinking processes behind verbalization is critical, but what about the actual speaking part of language? (Obviously, it's much more difficult to think of what to say as you're saying it rather than, for example, when typing a blog post leisurely.) Is cognition impaired for children that communicate through sign language? Or could there be a benefit to writing? Additionally, how does the content of communication matter? Would an intense disagreement/argument do more to further cognitive development rather than a series of agreements?