Why do we need to use RxAndroid with RxJava? What is the functional difference between them and actual use of RxAndroid and RxJava? I can't find proper answer for this.
RxAndroid can be understood as an extension to RxJava, that helps you to use it on Android platform easily. See the description on RxAndroid Github repository:
Android specific bindings for RxJava 2.
This module adds the minimum classes to RxJava that make writing reactive components in Android applications easy and hassle-free. More specifically, it provides a Scheduler that schedules on the main thread or any given Looper.
Note that when you're using RxAndroid, you should also use the latest RxJava version as a dependency:
Because RxAndroid releases are few and far between, it is recommended you also explicitly depend on RxJava's latest version for bug fixes and new features.