That's a interesting question, why do we feel the need to formulate thoughts, when we could just act on pure will, or think without the actual words. I would guess that it's because we need confirmation that the thought was actually thought and not just some random noise. Also we have the "thought steps", where you get a basic feeling for what you want to think, then you formulate it with easy associations about the goal of your thinking (what it will eventually lead to), and then you formulate the path between the start (the basic feeling) and the goal (what you want out of it), between them are images of what you wan't to think and other associations that can possibly change your direction so that you have to find a new goal with your thinking (and so on...could go on indefinatly until you want to stop the thought process). New ideas would be very hard to get without this kind of thinking, also we have to think before we speak, now this is often only about the start and the goal, the speaking in between comes naturally, and you allways use your experiances to avoid misunderstandings (that is kind of a background thinking while you speak), in between the speaking process are a feeling of what paths you should choose to get your meaning through.
tQySmA: because we want to do what feels good at the moment.. Sometimes it's thinking, sometimes not..
