The vendor of the available business will need to see this in order to ascertain whether you are right to buy. It will demonstrate whether you have the correct business acumen required and whether you are serious about the business. It will also be useful when it comes to the financial side of things, but more on this in a minute.
You should also be aware of what NOT to put in your business plan. The issue of future forecasts is a contentious one. While it's all very well attempting to make future forecasts, it's difficult to predict too far ahead. What you need is to look to the short-term future and then produce your plan accordingly. As the business plan then continues, you can modify the content as and when it's necessary. Long-term planning, however, will only prove to be a pointless exercise.