Apple has some restrictions in place to prevent access to LaunchAgents running in a user session context.
But you may want to load or refresh a LaunchAgent as part of your install without requiring the user to log out and back in.
I prefer not to require logouts and reboots in my installation packages. Where possible, I use munki’s unattended option so software installs silently and the user is not prompted.
After some experimentation, I came up with this hacky method of getting a LaunchAgent to load from a package being installed as root. If you have a cleaner way to accomplish this, please let me know. Update: Please see Per Olofsson’s comment for a much better method until I update this gist.