Mkinitrd creates file system images for use as initial RAM disk
(initrd) images. These RAM disk images are often used to preload the
block device modules (SCSI or RAID) needed to access the root file
system.

In other words, generic kernels can be built without drivers for any
SCSI adapters that load the SCSI driver as a module. Because the
kernel needs to read those modules, but in this case is not able to
address the SCSI adapter, an initial RAM disk is used. The initial RAM
disk is loaded by the operating system loader (normally LILO) and is
available to the kernel as soon as the RAM disk is loaded. The RAM
disk loads the proper SCSI adapter and allows the kernel to mount the
root file system.