what is the role of RxJava2CallAdapterFactory.create() in retrofit

Question

what is the role of RxJava2CallAdapterFactory.create()...

Retrofit retrofit = new Retrofit.Builder()
.baseUrl("https://api.example.com")
.addCallAdapterFactory(RxJava2CallAdapterFactory.create())
.build();

Answer 1

From description:

A call adapter which uses RxJava 2 for creating observables. Adding this class to Retrofit allows you to return an Observable, Flowable, Single, Completable or Maybe from service methods.

interface MyService {
    @GET("user/me")
    Observable<User> getUser()
}

Without it you cant describe interface as rx type