Debuggers are program developers' tools for tracing through their code.  These tools are used to validate that the program  follows the expected behavior.  With a debugger, we can trace our code line per line.  We get to see every instruction in action as it make changes to the registers and data stored in the memory.  In reversing, debuggers are used to analyze programs at its low-level. With what we learned about assembly language, the target compiled program, and a debugger, we are able to do reverse engineering.   

Besides the tools introduced in this book,  there are a lot of tools available in the internet that may have more or less features.  The point is that reverse engineering rely on the tools and we need to keep ourselves updated with the latest tool. Feel free to download other tools that you want to explore and see which one makes your reversing feel more comfortable.