This product is basically the first few chapters of the book. This allows you to share it with your players and for them to get a feel for the game, as well as create a character, without the need for the actual book. (Thus cutting down on costs to your table).
The book contains large chapters that are GM information, and players getting nosy, could potentially spoil the fun of discovery. Thus cutting down on the immersion of the game.
Threats (bad guys), as supposed to be new to the players, and they even name them, not having exposure to them before. There is also a great meta-plot at play, discovering why the world is the way it is, and how the players came to be mutants. This plot plays out over a campaign, and a player would spoil it for themselves to have preexisting knowledge of it.
The company, Modiphius, knows this and rather then gouging your table for more copies of the book (and tempt any player that happened to buy one with peeking ahead) , they provide this player aid for free.
This company is stellar and that they consider these things shows devotion to their customers and product.
5 stars, besides, it's free!