The risk of not doing a business case is you will take actions or make decisions without having thought through all the implications – not just the financial implications, but the implications for the overall business.
That could mean organizational implications, managerial implications, strategic implications and financial implications. All those things come into consideration when you’re doing a business case.
If you prepare a business case for a decision you are making, you are going to be able to look at it more objectively. In a business case, you build a set of assumptions. If you choose a path and you see early on that a key assumption is not turning out as you thought, you can revise the plan and make adjustments as you go.
A good business case allows you to see options well, so it gives you a road map to choose alternative paths if the perfect path is not available. If things aren't going as you hoped, the business case can help you choose the next best option.