Planning makes it very clear for us to prioritise our tasks. Time management is eased. Schedules are kept without any hazzles. Planning means saving of money, time and energy for indivuduals and organizations.
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.