The Scrum method does not explicitly state that a business case is needed. So why should you have one?
First of all, let me ask you another question. “Do Scrum projects have a business case?”
Well of course they do. Every project has a business case. A “reason why”, a “justification of”, a specific goal. Sometimes a business case only exists in the mind of the business executives, sometimes an elaborate analysis has led to a business case document.
As stated in my previous post projects tend to align to the measurement system and it is wise to align the measurement system to the business case. The same applies to Scrum projects. It does not help when the business case is not defined explicitly.
Within the PRINCE2 project management method the business case plays an important role. Unified Process also uses a Business Case. Scrum doesn’t have it, but if there is really good teamwork going on then probably the business case is clear anyhow.
So, does a Scrum project need a business case?
I would say yes. It plays an essential role to maintain focus on what is important. Even when teamwork is great it still helps a lot. It also can help managing the customers expectations.
At the same time I think the Business Case should be agile. It is of no use to spend too much time on analysis or describing the solution. That would not be agile and probably not effective. To me an Agile Business Case resembles a User Story. It describes the need of the customer and defines the boundaries of the solution/product, it does not describe the solution itself.