"Plans are useless; planning is indispensable," according to Dwight D. Eisenhower, then Commander of the Allied Forces in Europe during WWII. Now, you may be in total agreement with the first part of that statement, but you are really not convinced of the truth of the second part.
Once you have a plan in place, it's important that you follow it. If you're achieving your goals you should stick with the plan. If you are not achieving your goals then you will have go back, analyze your plan to find out what is working, what is not working and why it is not working. A plan is not etched in stone. It is subject to change. As time goes on, things change in this world and businesses like everyone else are subject to change. A good plan will reflect changes that a company has to make to keep it competitive and successful.