Why do you need to have a plan if the plan doesn't exactly match the reality? Are there any benefits to the planning process?
"In preparing for battle I have always found that plans are useless, but planning is indispensable" ~Dwight D. Eisenhower
The process of planning involves understanding the end goal, different strategies to get to that goal, understanding the situation and circumstances, understanding the availability of resources, the competitive positioning if it is a competitive situation. The outcome of the process of planning is a plan that tries to capture all these variables and comes with a reasonable action plan under the projected situation.
The plan is your best guess to get to your goals given the circumstances.
As soon as the plans are put in place they start to go haywire. The reality starts to look quite different. As many of the variables predicted really turn out to be different in reality. It would seem that the even the best plan starts to fall apart as soon as it starts to get implemented.
However, most successful people swear by the plan and the planning process. The reason is that while creating a plan the process of planning involved looking at all the different pieces of the situation. You become acutely aware of the situation and what factors might be critical to your success, what factors might be not as critical to the end goal. You also gain an understanding of the impact each of these factors is going to have on your final outcome. Hence, you are able to respond to the dynamic situation.
The plan also helps to provide a baseline for measuring your results. As you can start to compare your plans against the actual results. This will provide you an understanding that either the circumstances have changed much more than originally predicted or some other factors need to be incorporated in your plan. It is always a process of learning and adapting. Your plan is as good as your planning process and which is as good as your understanding of the overall situation.
That is why even if the plan might not look very useful during the execution but the going through the planning process is what is very useful.