Why have a model of the world? The obvious answer is to allow us to act and react. If we do live in a model of the world, what would be needed in the model?
We need our environment in the model and this we get from our sensory perception, especially the distance senses: sight, hearing and smell. What we produce is a space with us at the center. We have some idea of what lies in every direction from that center and the movements and changes out there. The environment nearer to us or things we are manipulating come from these same senses but also importantly from perceptions based on touch (pressure, heat, pain).
Then we need to model ourselves in this environment. Again touch is important. But mostly we model our bodies from our sense of balance and acceleration and from the perceptions of posture and movement that we create from joint and muscle signals.
So now we have a model of ourselves in our environment but that is not enough to allow us to act and react. We are missing something. We dont want something like a bunch of still photographs; we want a movie. We need a plot, a story line, a theme or a narrator. We need a structure that corresponds to the series of ideas: motivations goals plans decisions – actions. So it would not be surprising if the model included a theory of mind for our own behaviour.
We are assuming that we are dealing with a model. The physical tree is not actually in our model. Our model includes an item that models the tree. Our foot is not actually physically in our model. Our model includes an item that models our left foot. Our sitting down is not actually in our model. Our model of our body moving at sit in our model of a chair is being modeled. Forget about anything being physically present in our model; they are only represented in the model. A model is a model and what it models is what it models. They are not the same. There is reality and there is a model of reality not the same thing.
Maybe you can see where this is going. Our decisions are just like everything else. They are not physically in the model and so they are not taken in the model, they are being modeled in the model. What we have in consciousness about our decisions is simply what has been modeled about them and nothing more. The model has to be fairly accurate or it would not be of much use. When we physically make a decision there is a high probability that it will be modeled along with the modeling of the consequential actions. But like everything else in our perceptions, the highly detailed, fine-grained modeling is reserved for those things that are the focus of our attention. We have much more awareness of some of our decisions then of others.
When we introspect we are not getting to know directly what is going on in our brains, we are just focusing attention on our model of our mind (not focusing on our mind, focusing on our model of our mind). That means our model has more detail of the processes of our mind but it is still the model and not the real thing. We understand our own mind a little more clearly then we do other peoples minds but not that much more directly. We do not real know how we take decisions because our neurobiology has not progressed that far. What we know is how each for us models our own decisions.