Buying your business and looking for financing by a bank, angel investor, buddy? You need a business plan.
Selling a business? One of the features of a good for-sale process is for the seller to be able to show the buyer a plan for the future. It will help to wet the appetite of the buyer.
Operating a business? You should have a business plan that helps to guide decisions, initiatives.
This article by someone who lives business plans, Tim Berry, is worth the read. He gives great tips.