As soon as you open a business, you have to constantly evaluate how well you are doing according to the assumptions made in the business plan. If customers aren't coming in, you might need to change your business hours. Maybe your inventory should be different. Everything changes you know once you are really doing it. So you plan for it and once it happens you want to revisit all your assumptions because if your assumptions are wrong your finances are going to be wrong and we are all doing it for financial results, so you want to revisit how you are doing.
Maybe things are going better than planned, which is great, so you say, "How can I take advantage of this and do even better?" Another time to look at your business plan to see if it needs changes is after a big event or change. For example, here in Florida we have hurricanes. After a hurricane, something may have changed which greatly affects how your business is going to be done - positively or negatively. Look at the roofing business, which went crazy after hurricanes because there is so much demand for people to re-roof and have their roofs repaired. So people had to change and talk about the resources they need. They needed more help and things like that. All the businesses like restaurant businesses or hotel businesses, which cater the tourists, experienced a decline in business because tourists weren't coming. So whenever there is a major event that changes what's happening in your business, you have to look and plan for your recovery from that event whether it be positive or negative.
The worst things to happen in business are surprises both good and bad because if you can't capitalize on the good things, you lose opportunity, but you get better surprises if you can readjust your business. So the business plan will help at least to avoid surprises, to deal with surprises.
