What is the benefit of me creating a plan in advance for the coming year, or for the next 3 years?” To keep your eye on the prize!
You want to have concrete sales numbers for the next year broken down by month, so you can keep your focus on your destination. Having those numbers in front of you, will help you reach your goal.
Is this a guarantee that you will reach your goal? No, but it will definitely help you! Think of it as setting a destination on your GPS. Setting your desired location will help you reach it, whereas starting driving on the freeway with no direction will take you nowhere.