It's essential on some embedded boards as a first step because a well crafted initramfs can fit in a few MB of flash, and it's sometimes the case that the onboard bootloader can read the flash but has no other capabilites (well, aside from usually tftp).
So you read the kernel and initramfs from there, then load the real root FS from whatever storage you need to.
So you read the kernel and initramfs from there, then load the real root FS from whatever storage you need to.