It helps with figuring out long-term trends, but it's also an improvement even for a single snapshot-in-time decision. If your blood glucose is 150mg/dL and you're trying to get it to 100mg/dL, the difference between "150mg/dL and rising 3mg/dL/min" and "150mg/dL and falling 3mg/dL/min" is the difference between wanting insulin and wanting carbohydrates.