Gentoo Build Binpkg if you will Chroot is a collection of scripts, that ease managing chroot environments.
This is useful if you have any arbitrary need for chroots.
This might be also useful for building Gentoo's binpkgs if you have a number of similar systems and need similar binpkgs for them, you could build them in a chroot on a powerful/efficient/whatever-that-is system and then distribute amongst target systems. Alternatively you might have a powerful system, that could build various (different march, arch etc.) binpkgs for other systems.
To get started, read the Documentation.