The most important attribute/muscle for individual success is willpower.
Willpower is a muscle that can fatigue and get stronger.
The best way to strengthen willpower is to make it a habit.
As you exercise willpower, the muscle of discipline is exercised in your brain, making willpower easier to use in other places of your life.
Writing out a plan allows you to visualize inflection/pain points, which means less will power will be needed... meaning higher probability of success.