Upon installing the app (on android at least) it asks if you want to let it run in the background, which is obviously needed. It asks but you don't really have an option. I couldn't get past that part without clicking OK or whatever it was.
This is good for devices that don't have cell service like tablets. But I was trying to install it on my primary phone. I have a OnePlus 2 which doesn't support TMobile's Wifi calling/texting. I think I can disable my line in the DIGITS app, but the app itself continues to run in the background.
So being able to install the DIGITS app and being able to turn it on and off in situations where I only have Wifi would be useful.