There are two reasons to buy this book, along with its companion, Mastering the Art... Volume 2.
First, and most important, "Mastering" is an essential reference book on the French style of cooking. Whatever you're trying to make -- from simple things like chicken stock or scalloped potatoes or coq au vin to something that would try the patience of Job -- it's probably here, and with detailed, step-by-step instructions. Whether you follow the recipies literally or devise your own shortcuts, you'll know what's "right" and be able to make your own choices about what to do.
Secondly, it's a breakthrough book and a classic, capturing the state of French cooking and Americans' knowledge (or lack) at a particular point in time. In addition to the step-by-step instructions, the recipies are full of offhand comments about who taught Julia what and on the nature and source of the ingredients.
There are two aspects of these books which make them not for everyone. First, Julia brooks no shortcuts. Even relatively simple dishes can take some time. Second, the instructions are extremely detailed. This can be a virtue, but it can also be frustrating. A recipie can run several pages. This makes it a bit challenging to see the big picture of how the recipie is structured, or to find your place again once you've cleaned your knives and your hands (for the fourth time.)
That having been said, if you like to cook French and you have any interest in the classic recipies prepared the classic way, this book is indispensible.