if you're using ext4 or another Linux-first filesystem, you actually have more disk space than the tools tell you about
there's an extra percent or so that is hidden from the free-space reports and can only be accessed by root. stuff running as non-root users is told "can't do that, no free space" before it's actually true.
of course if all your daemons run as root, then the logs or whatever that are filling your disk to the breaking point will also fill the extra space. good distros will run each daemon as its own user because that's the modern way, but you may still run into trouble with your own custom services if you didn't set up a user for them
this means that if you are ever trying to free up disk space and everything is jammed so tight that it can't do that, sometimes you can get it unjammed by doing stuff as root