Do we ever need Observable.just(..) or Observable.fromCallable{...} as we already have Single.just(..) or Single.fromCallable{...}?

In RxJava2, we have Single as an Observable that will only generate one and only one data.

One could code

Single.just(...)
// OR
Single.fromCallable{ ... }

However, in Observable we could also do that

Observable.just(...)
// OR
Observable.fromCallable{ ... }

The different is in Observable, we'll have an extra onComplete callback once the one and only data is given. Nothing else.

Hence I was wondering do we ever need Observable.just(..) or Observable.fromCallable{...} since the Single version of it should suffice? Is there a scenario where Observable.just(..) or Observable.fromCallable{...} can achieve (or better than) but not with Single.just(..) or Single.fromCallable{...}?