Design controls are a way to prove that you have done everything according to the procedures outlined by the FDA. It ensures that everything is accounted for in terms of risk management and that it performs the way it is designed to. Not only does this help the manufacturer for any liability issues but it also allows for future modifications. The design control document should have clear diagrams (if applicable) and it should allow any person to understand how you came up with this device and the steps you took to make it.