The introduction of stream processing in Java 8 is a concrete example - it can auto-parallelize computations. It was not immediately clear to me why an identity value was needed in this function:

    http://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#reduce-T-java.util.function.BinaryOperator-

After reading up on monoids, it became clear. Note how the reduce() variant that does not take an identity value is allowed to return null!