Run filesystem

run is a special file system in linux
it is meant for runtime operations (which do not need to be persisted on storage ?)

The /run mountpoint is a tmpfs mounted during early boot, available and writable to for all tools at any time during bootup
it replaces /var/run/, which should be a symlink

directories where it gets mounted :
/run
/var/run

The /run directory is the companion directory to /var/run. Like /bin is the companion of /usr/bin
Daemons like systemd and udev, which are started very early in the boot process
have with /run a standardized file system location where they can store runtime information.
/var/run was the wrong location, and most distros either symlink or bind mount them to each other. They are not intended to be separate directories; /run is the correct location, /var/run is kept around to support legacy software.
Debian/ubuntu etc  /var/run  is symlinked to /run

use case
it also gets used -I think- a bit like the ramdisks of old
=> zulucrypt / or LUKS mounts volumes here
or apps use this ‘trick’ as well
propabarly because they know it it is always ther

https://askubuntu.com/questions/303120/how-folders-created-in-var-run-on-each-reboot