Unconscious performance allows mental taks to run much more quickly, and we have seen many indications that speed is a desirable feature in our intellectual functioning. The routine allows a withdrawal of attention from the task, and this allows you to devote your attention instead to other, more pressing matters.
What makes this withdrawal of attention possible? When you're learning a new task, you need to monitor each step, so that you'll know when it's time to start the next step. Then you need to choose the next step and get it started. Obviously, this gives you close control over how things proceed, but this need for supervision of the task makes the performance quite demanding. After some practice the steps needed for the task are still there, but you don't think about them one by one. That's because you have stored in memory a complete routine, and all you need to do, is launch that routine.
Practice allows you to perform a task while allocating your resources elsewhere. The invisibility of our mental processes is often a good thing; it allows us to keep attention focused on the things that matter without having to worry about the details.