On Arch Linux, and probably many other distros
/run is a new tmpfs, and /var/run symlinks to it.
With Lighttpd you might have a fastcgi socket defined something like "/var/run/lighttpd/sockets/mywebsite.sock".
This won't work anymore as after each reboot /var/run is an empty directory and lighttpd won't start, /var/log/lighttpd/error.log will tell you:
2012-03-16 09:21:34: (log.c.166) server started
2012-03-16 09:21:34: (mod_fastcgi.c.977) bind failed for: unix:/var/run/lighttpd/sockets/mywebsite.sock-0 No such file or directory
2012-03-16 09:21:34: (mod_fastcgi.c.1397) [ERROR]: spawning fcgi failed.
2012-03-16 09:21:34: (server.c.945) Configuration of plugins failed. Going down.
That's where this new tool tmpfiles.d
It creates files and directories as described in the configs, and gets invoked on boot. Like so:
$ cat /etc/tmpfiles.d/lighttpd.conf
d /run/lighttpd/sockets 0700 http http