The promise is that it informs you quickly about flight delays, flight cancellations and gate changes. In my limited experience, it didn’t work satisfactorily for a flight delay of a few hours. It could not provide any reliable updates.
It’s a nice app and service, but I wouldn’t trust all those reviews that are like “I knew before the aircraft pilot knew”. It has its own limitations.
Yeah, the most notable "use", not necessarily "value", is when the airline is still prevaricating over the delay, you're approaching boarding time and you can see from ADS-B that the inbound aircraft hasn't even begun initial descent.
I don’t see any value in knowing before the pilot knows. I’ve mostly flown American the past few years and with their app I get updates about delays and gate changes on my phone just fine. I suppose there might be some advantage to getting the notification a bit earlier, but I doubt that they can reliably give information faster than the airline itself.