A main use case I can think of, is if you are not at home. And you don't use PiHole + PiVPN. Then it would be convenient for you if you put this app on your phone and just use it like a 'local' PiHole (but it's also working at an app level).
I don't believe you'd get your PiHole itself to use this service, it's not a public DNS service that you can get your PiHole to point at.
As for NextDNS, are you asking, would you get your PiHole to use your NextDNS as an upstream? You could do that, there isn't a huge advantage to it though... as PiHole is already doing the work for you. Your main motivation to do it anyway would be the consistency factor.
I don't believe you'd get your PiHole itself to use this service, it's not a public DNS service that you can get your PiHole to point at.
As for NextDNS, are you asking, would you get your PiHole to use your NextDNS as an upstream? You could do that, there isn't a huge advantage to it though... as PiHole is already doing the work for you. Your main motivation to do it anyway would be the consistency factor.