When you formulate a thought or a sentence you compress vast amounts of data into something that's understandable in more general context. Once you've done that compression, you interpret that generalization once again as you vocalize it. This reinterpretation is powerful and may trigger some ideas that you haven't had before.
Besides, try doing math or debugging some code without any conscious thoughts.
I agree most of the time it may be a broken record and useless, especially if you repeat the same patterns over and over again, but I'm quite certain you don't know what you are going to say until you've said it (out loud or not).