I don't know if I can quote a part of my hub "Why do the others succeed while I fail?" here because it answers your question.
In short, planning is important because it is a gain of time and effort. Doing tasks according to your priorities makes you avoid turning around searching for a goal, thus, a lot of non useful work will be avoided too. Knowing what to do, how and when is the main purpose of planning. Somehow, It is a shortcut to success.
Welcome on HubPages Maai :)