To the extent that consciousness is useful, it is useful in small quantities, and for very particular kinds of tasks. It's easy to understand why you would not want to be consciously aware of the intricacies of your muscle movement, but this can be less intuitive when applied to your perceptions, thoughts, and beliefs, which are also final products of the activity of billions of nerve cells.
In the wake of the Aurora movie theater shooting, many people had the same questions: What kind of derangement is indicated by the horrific acts of James Holmes? What is wrong with his brain? How will his mental state play out in the courts?