Say you're at work or out of town, and the hose behind your washing machine splits. Instead of coming back to a wet floor, you get a notification on your cell phone that water is running where it shouldn't be. The device can turn the water off by itself, or you can shut it off with one tap in the app. You can check on your water from the app while you're away, and a small leak is more likely to be caught early.
Which unit fits your home depends on the size of your water line, your water pressure, where the line comes into the house, and whether there's a good spot with power and WiFi.