HANA native webdispatcher service vs external Web Dispatcher

As per Admin Guide (SPS09) SAP recommends installation of external Web Dispatcher or another HTTP load balancer (p544):

Doesn't webdispatcher come as a native HANA service? What would be the difference between the two and do we use both at the same time?

My guess is that we need the external dispatcher to protect against scenarios where one of our hosts crashes but the external dispatcher is not effected and simply redirects the clients to the other active server (please correct me if I am wrong...) But do we still use both internal and external web dispatchers at the same time?

Thanks!