Writing can help to clarify thinking. I remember an old German professor who wanted us to see, say, hear, and think in German. It worked so much better, having the words in front of us, saying, and thinking them. We really learned to think in German.

I think that ygrec has hit on this too. By writing you are looking at words and feeling things and seeing things more deeply than you do in just saying them. My husband and I fell in love when he was in California and I was in Michigan. We wrote long emails to each other daily. We expressed so many deeper feelings about life and what was happening because we were writing instead of dating and chatting about the movie or weather or sports, etc.

Writing can also be a tension release, if you don't have someone to talk to. My best friend does a lot of email drafts that eventually get deleted, never sent. If she is unhappy with someone, she tells them off in an email. Just never sends it--it stays in the draft box until she's gotten over whatever it was, and then she deletes it.