A bit of a mystery remains! What exactly is this initramfs or initrd image for? Why is it there?
Firstly, using this feature is a choice – the config directive is called CONFIG_BLK_DEV_INITRD. It's on and hence set to y by default. In brief, for systems that either do not know in advance certain things such as the boot disk host adapter or controller type (SCSI, RAID, and so on), the exact filesystem type that the root filesystem is formatted as (is it ext2, ext3, ext4, btrfs, reiserfs, f2fs, or another?), or for those systems where these functionalities are always built as kernel modules, we require the initramfs capability. Why exactly will become clear in a moment. Also, as mentioned earlier, initrd is now considered an older term. Nowadays, we more often use the term initramfs in its place.
