I would say that the main benefits of FP programming are

1. Its easier to define your own abstractions in libraries. Even advanced stuff like async programming can often be put on a library.

2. Languages like Haskell and OCaml have really cool type systems. Its a killer feature, IMO.

I wouldn't put that much emphasis on the purity aspect. In Haskell the reason for the purity to allow for lazyness and similar languages (Ocaml, F#, etc) are perfectly OK with strict evaluation and mutation.

That said, for mobile apps having a language that operates well with the rest of the ecosystem is probably a bigger concern. Its easier to choose your own language when you have more control over the running environment.

Intentionally or not, the thought process FP languages generates will creep its way into your programs, and then probably into the way you would write object-oriented programs.

FP has been hard to realize on mobile mostly due to the limited choice of languages available.

Yeah, I agree with this. However, I could still see a lot of merit in writing the server that the mobile application talks to in an FP language.

RxJava brings lot of FP idioms to Android, and it's great

Are you developing for iOS? Swift seems to introduce a lot of FP concepts.

Mostly Android. I was thinking about maybe using Scala just to make something meaningful while learning FP.