Organizing your day and writing down what you have to do is a key component getting things done.
When you organize your day or week before hand you know what you have to do and when you have to do it.
So while organizing your day/week you can add what you want/ have to do which can help you know how much free time you have and what you can do.
Also there is an amazing satisfaction to checking down what you have to do, also it could be a motivating factor to see you have stuff to do.