It comes in very handy for queen breeders who refer to the chart when looking for young eggs that are between 1 to 3 days old to use for grafting. The age of the egg is revealed by it's position in the cell (upright, slanted downward, etc)
I did a review of the book The Biology of the Honey Bee written by Mark L Winston. It's a really good book and has a ton of information which you will find useful. It's well worth the purchase.