These are components that run inside-the-network but are not managed by Kubernetes. There are many reasons to run such components. They could be legacy applications that have not be Kubernetized yet, or some distributed data store that is not easy to run inside Kubernetes. The reason to run these components inside-the-network is for performance, and to have isolation from the outside world so that traffic between these components and pods can be more secure. Being part of the same network ensures low-latency, and the reduced need for authentication is both convenient and can avoid authentication overhead.