hostNetwork setting to false doesnt allow copy to hostPath

Question

I have theoretically gone through hostNetwork definition provided by Kubernetes. For the below pod spec

cat hostpod.yaml 
apiVersion: v1
kind: Pod
metadata:
  name: test-pd
spec:
  #hostNetwork: true
  containers:
  - image: ubuntu
    name: test-container
    command: [ "/bin/bash", "-c", "--" ]
    args: [ "while true; do sleep 30; done;" ]
    volumeMounts:
    - mountPath: /test-pd
      name: test-volume
    securityContext:
      privileged: false
      runAsNonRoot: false
      runAsUser: 0
  volumes:
  - name: test-volume
    hostPath:
      # directory location on host
      path: /usr/local
      # this field is optional

And trying to write to a file

root@test-pd:/test-pd# touch newthree.txt
touch: cannot touch 'newthree.txt': Permission denied

But this works when I set hostNetwork:true.

could I get some inf on how this actually works and why hostNetwork is required.