Just imagine - you happen to be working on really old project. 10 years at the very least. That's where a perfectly documented system would make really a favour.
Each one of us can find himself in situation like that, even on the project that you are currently working one in 10-15 years.
Information flows trough an analist in charge of the project. Some part of it are vital for developement.
The other parts gather value as the years pass by. But the moment you need this info it can be next to impossible to turn back time and go 15 years back from now.
In the talk I sum up my experience in dealing with documentation to figure out what exact information parts is the ones that make documents valuable and efficient in future.