I'm not sure if it also shows some infrastructure graphs, but I'm talking about knowing if links are up, how they are firewalled, where the config for each thing is, etc.
When you host tens of services on hundreds of machines, this information is hard to get a grasp on, no matter what you do or how well you documented everything, because it takes a while to read through it.
Then whenever a significant change is required, you need to call back your "expert". New location? xk$, etc.
But I think the biggest concern is that suddently the owner does not understand how his reversation system works. He used to be able to call Joe and know what's going on...