Many images that are required for a node are pulled from external registries (docker.io, gcr.io, registry.k8s.io).

This can cause new nodes to not be able to properly join the cluster if one of those registries is having issues. (Example: Cilium images could not be pulled due to an issue that caused HTTP 500 errors on docker hub)

The use of an internal mirror for these essential images can prevent these kinds of issues from affecting a cluster, and also improve pull times.