Unscheduled time sucks up all the time
If you don't schedule your time, you will spend it wandering aimlessly around, probably not accomplishing anything that you wanted to.
When I allocate a specific amount of time for a task, or when I have a deadline that must be met, the task has a tendency to get done. When I just start working on something it will possibly get done, but more likely I will procrastinate, get side tracked, expand the scope, etc. In other words, to get things done, schedule time to work on it and set a deadline.
According to the famous law of Parkinson, every kind of work tends to fill the time we set aside for it, like air fills every corner of a room (Parkinson 1957). How to Take Smart Notes, page 143 125.002 Books - How to take smart notes