What are the use cases of journalctl --flush ?

Hi, I have already read man page of journalctl and made some experiments with the command.

All the efforts made me more confused. So, I would like to beg some help here.

My system is Ubuntu 20.04.3 LTS.

My procedures:

1.     Create a .conf file for systemd-jounald.service with following content:
[Journal]
Storage=volatile

2.     Restart systemd-jounald.service

sudo systemctl restart systemd-jounald.service

3.     Check the status of journald and ensure it is writing log into memory.

systemctl status systemd-jounald.service

The result is like:

Runtime Journal (/run/log/journal/...)
which informed me that .conf file worked well.

4.     Comment out the Storage option
[Journal]
#Storage=volatile

5.     Restart journald

sudo systemctl restart system-journald

6.     check status again and you will see that flush already been done

systemctl status system-journald

with feedback including

Time spent on flushing to /var/log/journal/machineID/ is 282.991ms for 1483 entries.

which means flush had been done.

Now that the operation of flush can be done automatically when you switch from Storage=volatile to #Storage=volatile, why do we still need journalctl --flush?

I believe there are some reasons for the existence of this switch for journalctl.

Hopefully, I can find some help here.

Thank you!