Unshare with --mount-proc creates a new mnt namespace

I am a little confused about what --mount-proc does when used with unshare command. When I use unshare -fp --mount-proc bash, I notice that it results in both a new PID namespace and a new MNT namespace. Given that I am in a new MNT namespace, I tried unmounting one of the loop devices and noticed that this was not reflected in the parent MNT (the mount still shows when I run df -h in the parent namespace). Now, I went one step ahead, and, from within the new MNT namespace created a new root using pivot_root, and unmounted the original root. As expected, the new namespace now has a different root. The parent namespace still has the original root directory. My Question: If I can achieve creating a new MNT namespace without using the -m option in the unshare command, and also achieve creating an isolated root directory for the new process, what different purpose does the MNT namespace serve? I would be grateful for any guidance from the experts. EDIT: I have modified my original post, which said that changing the root in the new namespace also changes the root in the parent namespace. I am no longer able to reproduce this. But my question on being able to create a new MNT namespace with just the --mount-proc option and without using -m with unshare still remains.