Dave Baldacchino wrote a post at his blog the other day focusing some attention on these so I dug this information up.
I wrote the following documents in 2004 while I worked for Wimberly Allison Tong & Goo (WATG). It's still relevant today though some improvements in visibility control (such as using filters for managing grids instead for example) have reduced the necessity for using them, somewhat. Their facility at managing consistent views of the model for multi/many story projects probably remains the most compelling reason to use them.
Ironically I didn't focus on that in this document, I guess I took that bit for granted. What I did do is focus on their role managing the visibility of datum in views. There is a sample project file on my site. Below this first embedded document is another that is a guide to experimenting with the file.