@dhawalschumi the only difference is that you can deal with an observable right away. The CouchbaseCluster is actually just a sync wrapper around CouchbaseAsyncCluster, so functionally there is no difference. Both are available and you can pick what fits your needs.
The two threads you found there are internal request and response handler threads, nothing that needs your concern directly as an app developer (unless you want to learn the internals). There is no overhead/gain by choosing one or the other because of the reasoning written above.
Does that answer your question?