There are many ways to use the Wine Grape Varietal Table: for looking up what grape goes into what wine, how a grape relates to others in terms of body, acidity, varietal characteristics, etc. or just browsing the world of wine grapes. Here's one possible scenario:
While enjoying a bottle of Sancerre for example, the question arises: "what grape is this made from?"
Look up Sancerre in the White Wines section of the accompanying Wine and Grape Indexes to learn it’s made of the Sauvignon Blanc grape:
To help locate Sauvignon Blanc on the Wine Grape Varietal Table, first look in up it the Grape Index.
Find Sauvignon Blanc on Row E and learn about its typical varietal characteristics, the countries it's grown in and other wines it goes into: