If you've built your own kernel with all necessary storage-controller and file-system drivers built in, then you may have no need of an early userspace environment. However, if you want to do anything non-trivial with your root file system (LVM, LUKS, etc.), then you need an early userspace to set up and mount it. The traditional mechanism for this is initramfs, but building and maintaining an initramfs image is awkward and tiresome. Initramfs is a sledgehammer when, nine times out of ten, all you need is a screwdriver. This guide details a method of booting into an early userspace environment located in an ordinary file system on a physical disk partition, where an init script in this environment in turn sets up and mounts the real root file system and pivots into it.
In order to employ this method of booting your system, you will need a traditional (non-LVM) disk partition containing a file system that your kernel can mount without