Plain definition
A business case explains why a decision is requested, what alternatives are available, what the preferred option needs, and how success will be judged. It is broader than a financial model. The model estimates numbers; the case connects those numbers with customers, operations, people, timing, risks, and the owner who will deliver the result.
A useful case treats doing nothing as an alternative, not as empty space. It also distinguishes a benefit that is expected from a benefit that is evidenced, and a decision that management controls from an external condition it merely assumes.
Illustrative example
Suppose a company proposes its own delivery fleet. The case compares buying vans, leasing them, outsourcing delivery, and retaining the current provider. It sets out capital cost, driver availability, insurance, customer-service effect, expected delivery volume, and the break point at which owning vehicles becomes cheaper.
The financial model may favour buying at high volume, but the case shows that the company has no manager ready to run fleet operations and demand is uncertain. Management might select a short lease or pilot instead. That is not a failure of the model; it is the business case making the operating constraint visible.
A case is not strengthened by more pages. It is strengthened when the requested decision, evidence, uncertainty, and accountable owner are easy to find.
Why it matters
A narrow financial return can hide the fact that a company lacks the people, process, permission, or customer evidence to achieve it. The business case prevents management from approving a number without approving a credible way of producing that number.
Practical consequence
Write the decision request in one sentence. Set out the credible alternatives, including doing nothing, and state what each would cost, change, and require. Name the benefits, assumptions, risks, dependencies, and delivery owner. Identify the few facts that management must verify before approval, and define the measures and review date that will show whether the case was achieved.