I’ve enabled full hard drive encryption on all of my devices. The only exception
is my homeserver (mainly running Nextcloud), where all of my personal data is
stored. I’m the only user and have chosen a very strong root- and user password.
From what I’ve researched, the only person who can see my data physically is the
super user (aka. me), but if someone else doesn’t have the password, they can’t
read anything critical and my personal data are safe from the eyes of others.
Is that correct? If it is, why does LUKS exist?
You are relying 100% on physical security. If nobody can take your drive or physically operate your computer, it is safe.
If I could boot a USB stick on your machine, or pull the drive, accessing your data would be trivial.