As you know, your brain is core to all of your thoughts, all of your actions and most fundamentally to learning what to do in new situations. Your brain takes in all the information from your senses (your eyes, ears, skin etc.) and makes meaning from them so that you can navigate around the world and take action.
You might not think of it this way, but the main purpose of the brain is to learn patterns and to use those patterns to predict what actions to take next. The more clear a pattern is and the more repetitions, the easier it is for your brain to make predictions and the more automatic your actions become.
Think back to the first time you learned to drive a car (if you learned to drive a manual transmission that can be an even better metaphor).
When you were first learning to drive, how much effort and concentration did it take just to pay attention to the mechanics of driving?