Do you need any book to play an RPG? No. You could make your game with your own rules. Why do most people not make their own? Because its a ton of work. You have to play test, look for problems, find solutions, and balance the whole experience. Timeline Fracture does the work of balancing and streamlining the use of multiple d20 books and provides a rich, diverse setting so you can just enjoy playing with your friends.
Part of the beauty of the d20 system is that once you learn how to play one d20 game, you know the basic rules for every other d20 game. However, companies that make their own d20 games tweak or change a few things to better suit their games. These changes from game to game become problematic when you mix them together. Most of these problems are fixable, but it takes time and effort to resolve them. When you encounter these problems during a game, it can slow it down to a halt.
Aside from the mechanics, Timeline Fracture offers a coherent world for your game to exist. If you say, "Fantasy", people may think Game of Thrones, Narnia, Lord of the Rings or King Arthur. All of these have fundamental differences in style, tone and setting. The same is true with sci-fi (Star Trek, Mass Effect, Star Wars) and other genres. Timeline Fracture defines each land in the New World, so every player understands what it offers.
Timeline Fracture identifies these differences and problems, offering you solutions before your game even begins.