Your IHD tells you how much your energy is costing over a period of time.
By having access to your smart meter readings, the IHD is also able to show how your current usage compares to your historic usage; brand new smart meters need a bit of time to build up a picture of your usage to do this.
IHDs help you monitor your energy usage, and can be used to see where you might save on energy by changing how you use energy, or by investing in efficient appliances.