The purpose of the Business Case may be one of several things: In the case of a project initiated by the client, it defines the project to the contractor and communicates the desired outcome of the project.
In the case of a project initiated by the contractor, the Business Case is the document which he uses to sell the project. It should clearly list the alternatives (and consequences). One alternative is ALWAYS "do nothing", which is not always a bad thing! I was hired by Denny's on a six-month contract to modify their point-of-sale, and after examining it my recommendation for the first project I was assigned to was "do nothing, the system will break if what you're proposing is implemented". I felt sure that I had just thrown away 50 grand, but as it turns out it was valuable advice which they followed, and they asked me to take on another project. That 6-month contract turned into two years. In the long run, working in the best interest of the customer is always in your own best interest as well.
It may be the case that you're developing a project on speculation... that's a little outside the scope of "Sponsored Project Development", but it fits into this development cycle as well. However, you'll have to take on the role of the Client yourself, or enlist someone in the field to act as your consultant and guinea pig. In this case it's sometimes possible to trade expertise for a beta of the finished product.
Originator: The client, often with professional assistance.
Deliverable: A Business Proposal document. (template)
Phase 1. Identify a Need Phase 3. Business Requirements