trez parent
some langages might not have an appropriate library, some might want to not have heavy processes on their device (mobiles). We also think that's easier to use as there is nothing to install. That mainly depends on your case.
I agree that there might situations where it can be useful, but:
1) Mobiles have pretty good CPUs. I think uploading and waiting for response would be slower and less reliable.
2) If the mobile user doesn't have an internet connection, the app won't work.
3) As a developer, I would be dependant on an external service, that could stop working someday.