quoted

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Is there any document which explain what "backing device" used for?

Have a look at the initramfs doc to get brief idea:
http://www.mjmwired.net/kernel/Documentation/filesystems/ramfs-rootfs-initramfs.txt

From what I've understood, is that backing device/store is the device

onto which _committed_ data is actually written. This might be a block
device like a hard-disk, flash mem or even the RAM (used by in-memory
file-systems).

CMIIW.

HTH,
-mandeep