From the Wikipedia page on the Filesystem Hierarchy Standard:
Modern Linux distributions include a
/rundirectory as a temporary filesystem (tmpfs) which stores volatile runtime data, following the FHS version 3.0. According to the FHS version 2.3, this data should be stored in/var/runbut this was a problem in some cases because this directory isn't always available at early boot. As a result, these programs have had to resort to trickery, such as using/dev/.udev,/dev/.mdadm,/dev/.systemdor/dev/.mountdirectories, even though the device directory isn't intended for such data. Among other advantages, this makes the system easier to use normally with the root filesystem mounted read-only.
So if you have already made a temporary filesystem for /run, linking /var/run to it would be the next logical step (as opposed to keeping the files on disk or creating a separate tmpfs).
