Small decisions can have a huge impact on final design cost. As the design progresses these values drop. In many ways these correlate to the design risk, but are more relaxed. In very simple terms the cost of the project can vary when the design does, risk is a good indicator of how much the design may change.
It is unlikely that a designer will pay much attention to this metric unless they are cost constrained. However they should be aware that a manager or project sponsor will weigh decisions to kill projects using this scale. Projects will be halted if the design exceeds, or has a real chance of exceeding the budget. And then time and effort will be directed to projects that have a greater change of providing a dependable Return on Investment (ROI).