The app provides information on my flight, my mileage records and can even track my checked bags.
You can get other information like Delta Partners. If you Park and Fly, there's a nifty tool that reminds you where you parked.
If you are in a multiple flight journey, you can track status of each segment.
Airport maps are also available.
Details also include the gate information for your flight.
If you are a regular passenger of Delta - go ahead and download the app from the iTunes store.