The purpose of a business case is to justify the project expenditure. This is done by identifying the business benefits that the project is going to deliver.
This is done by identifying the business problem to start with. As projects are usually undertaken to solve a particular business problem or opportunity. This is to understand the purpose of the project, the timeframes involved as this will act as the foundation of the project.
The next step is to identify alternative solutions and to get an understanding of the business problem or opportunity in depth. Then identify the solutions possible and choosing the right solution to solve the problem. Once the optimum solution is chosen, then ways to implement is chosen. The project business case will describe these steps and allow for easy understanding of the project by all its stakeholders.