Book excerpts:

Linux From Scratch (LFS) is a project that provides you with the steps necessary to build your own custom Linux system. And all of the instructions to create an LFS system are provided in this book.
The most important reason for LFS's existence is teaching people how a Linux system works internally. Building an LFS system teaches you about all that makes Linux tick, how things work together, and depend on each other. And most importantly, how to customize it to your own taste and needs.

One of the key benefits of LFS is that you are in control over your system without having to rely on somebody else's Linux implementation. You are in the driver's seat now and are able to dictate every single thing such as the directory layout and boot script setup. You will also know exactly where, why and how programs are installed.

Another benefit of LFS is that you can create a very compact Linux system. There have been already a system that was just enough to run the Apache web server; total disk space usage was aproximately 8 MB. With further stripping, that can be brought down to 5 MB or less. Try that with a regular distribution.