The arch linux install has a little wrapper around chroot, used to configure the installed system without booting it.
This is frequently very convenient if you want to install Gentoo by compiling everything from sources on a cheap and small computer, e.g. one with some Intel Atom CPU.
Instead of compiling anything on the resource-constrained computer, you install a fresh Gentoo system for it, in a chroot environment on a fast desktop computer, which supports a superset of the ISA of the small computer, so you can still execute the programs intended for the target computer.
Then you just copy the installation result over the SSD/HDD of the destination computer. If you have many identical computers, you can copy the installed Gentoo over all of them without any problems, removing the need for multiple installations.
If desired, you can keep the chroot environment with the installation result and perform any later updates on it. Then you synchronize the updated Gentoo from the chroot with the one or more target computers.
I just wish the script could figure out a BTRFS drive without me manually mounting volumes :(
for btrfs, if you use a consistent volume mapping on your systems, its pretty easy. in arch setup i typically enable ssh and have a pretty simple set of bashisms for target device, compression, and mount points. then its copy-pasta since its a boiler plate for when i need to recover or fresh install
not glanced at manjaro so not familiar with its install-methods