I have seen this option in zram-generator.conf but I have never looked into it. It seems to be a measure for Zram to write the incompressible pages into a storage device to free up space in memory.
writeback-device=
Write incompressible pages, for which no gain was achieved, to the specified device under memory pressure. This corresponds to the /sys/block/zramX/backing_dev parameter.
Takes a path to a block device, like /dev/disk/by-partuuid/2d54ffa0-01 or /dev/zvol/tarta-zoot/swap-writeback.
If unset, none is used, and incompressible pages are kept in RAM.
I guess if someone wants to use zram, it won’t hurt to add a little chunk of disk space for this purpose.