I got used to using the shutdown command like:
shutdown -t now
So when I started wanting to use reboot in the same way I was confused at first because to reboot now you need to use:
reboot -n
I looked more into and I noticed reboot could also be used to shutdown the system with:
reboot -p
What is the history behind these two commands? Did one come out before the other and are they both on most systems historically?