What is initrd why is it required ?

1. The kernel almost certainly will have been passed an initial RAM disk image (usually called "initrd") by the boot loader.
   
2. This is needed to provide needed device-special files in /dev, as devices are now dymanically created by the udev daemon which is only started during the middle of the boot sequence.
3. /dev will be an empty mount point, missing the needed stdin, stdout, stderr, null, random, and other devices.
4. The initrd may also contain some needed device drivers (e.g., for SCSI controller or SATA chipsets)
5. The initrd is uncompressed into RAM and mounted temporarily as the root of the filesystem (/). Then the file /linuxrc is run, possibly loading those device drivers.