It is common practice to create Business Cases for new products or major features. The makes sense, of course: Developing a feature creates direct and indirect costs. Therefore, business management expects a Return on Investment (ROI). However, when product managers see themselves as the experts, and they have a great idea, how can business people not understand the benefit of it? So the Business Case must supposedly convey exactly what business management needs to approve it.
Ian Lunn states why this is wrong in this blog post. A good Business Case must show accurate, true and complete information. Ian Lunn has a nice picture showing the difference between common (bad) Business Cases and ideal (good) Business Cases.