There are many reasons why somebody would want to read this book. The principle reason being to install an LFS system. A question many people raise is "Why go through all the hassle of manually building a Linux system from scratch when you can just download and install an existing one?". That is a good question.
One important reason for LFS' existence is to help people learn how a Linux system works from the inside out. Building an LFS system helps demonstrate what makes Linux tick, and how things work together and depend on each other. And perhaps most importantly, how to customize it to your own tastes and needs.
A key benefit of LFS is that you have more control of your system without relying on someone else's Linux implementation. With LFS, you are in the driver's seat and dictate every aspect of your system, such as the directory layout and boot script setup. You also dictate where, why and how programs are installed.
Another benefit of LFS is the ability to create a very compact Linux system. When installing a regular distribution, you end up with several programs which you are likely to never use. They're just sitting there wasting (precious) disk space. It isn't difficult to build an LFS system less than 100 MB. Does that still sound like a lot? A few of us have been working on creating a very small embedded LFS system. We successfully built a system that was just enough to run the Apache web server with approximately 8MB of disk space used. Further stripping could bring that down to 5 MB or less. Try that with a regular distribution.
We could compare distributed Linux to a hamburger you buy at a fast-food restaurant - you have no idea what you are eating. LFS, on the other hand, doesn't give you a hamburger, but the recipe to make a hamburger. This allows you to review it, to omit unwanted ingredients, and to add your own ingredients which enhance the flavor of your burger. When you are satisfied with the recipe, you go on to preparing it. You make it just the way you like it: broil it, bake it, deep-fry it, barbeque it, or eat it tar-tar (raw).
Another analogy that we can use is that of comparing LFS with a finished house. LFS will give you the skeletal plan of a house, but it's up to you to build it. You have the freedom to adjust your plans as you go.
Another advantage of a custom built Linux system is security. By compiling the entire system from source code, you are empowered to audit everything and apply all the security patches you feel are needed. You don't have to wait for somebody else to compile binary packages that fix a security hole. Unless you examine the patch and build it yourself you have no guarantee that the new package was built correctly and actually fixes the problem (adequately). You never truly know whether a security hole is fixed or not unless you do it yourself.