I don't have a copy of that book, but I can explain why a two-step process might be desirable if you're building a complete distro.
When building packages, it sometimes happens that properties of the build system creep into the binaries that you build. There are library paths, configuration variables etc, etc. At Sun/Oracle the standard process for building Solaris is that you need to build biweekly build 17 on a system that's running biweekly 16 (for example).
For example, there are many fields and structures in header files, you need to make sure that you're building the binaries using the header files that will be delivered on the system. But some of the packages you are building deliver their own header files. And those headers are used by other packages.
Hopefully I've given you an idea of how the system you are building can subtly depend on the system used to build it. So you would usually want to build on a system that is as close to the system you're building as possible.