JChroot represents a replacement to the arch-chroot previously used in JuNest. The main reason we have decided to remove arch-chroot is because it was not meant to be a portable script. The arch-chroot script is usually used during the installation of Arch Linux distro and it contains some commands (like unshare) that are not common on some old distros.
Furthermore, the arch-chroot belongs to the package arch-install-scripts which contains few dependencies. So, at the time the arch-chroot was part of the JuNest image, the image was bigger few tens of megabytes more.
JChroot takes just the main part of arch-chroot, namely, the mounts of the primary directories (i.e. /proc, /sys, /dev and /run) and the classic chroot executable. So, with JChroot we have a small and portable script that, also, drastically reduced the size of the JuNest image.