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.
Another point to bear in mind is that you need to keep the business plan relatively simple. Don't use fancy business jargon or clichés, since this will only cloud the important issues. In particular, if you are presenting the plan to a vendor or bank manager, you will need to keep the plan on point and free of over-fussy language and business speak. Present the facts in a concise, straightforward manner, and this will result in a plan that's both accessible and plausible.