In a book on how we think, Roger Martin suggests that:
Thinking – especially thinking in words and sentences – is a form of internal communication. In thinking, you-in-the-present communicates with you-in-the-future. But though thinking is a private and covert activity, it is influenced by external interactions – in particular, by how you communicate with others. Communicative patterns become mental habits. The implication is that counterproductive – closed, oblivious, disconnected, narrow, hermetic, rigid – ways of communicating are thereby internalized and become counterproductive ways of thinking.
And you didn’t believe that learning to write well was important? If Martin is correct, then the ability to communicate well correlates strongly with the ability to think effectively.