Interface indicating a operator implementation can be macro-fused back to Observable in case the operator goes from Observable to some other reactive type and then the sequence calls for toObservable again:

Single<Integer> single = Observable.range(1, 10).reduce((a, b) -> a + b);
Observable<Integer> observable = single.toObservable();

The Single.toObservable() will check for this interface and call the fuseToObservable() to return an Observable which could be the Observable-specific implementation of reduce(BiFunction).

This causes a slight overhead in assembly time (1 instanceof check, 1 operator allocation and 1 dropped operator) but does not incur the conversion overhead at runtime.