I think this was mostly pushed through by sysadmins annoyed at getting alerts from new applications that didn't mean anything to them.
Subject: Disk usage high
There is a problem in cluster ABC.
Disk utilization above 90%.
Providing a link to a runbook makes resolving issues a lot faster. It's even better if the link is to a Wiki page, so you can edit it if the runbook isn't up to date.