[manigandham]

The reason is the battery. These devices can be powered off and save state, like an SSD.

[piotrkaminski]

But in the context of the OP, presumably devices in a data center would never be powered down on purpose to save energy? In which case it seems that battery-backup DRAM would work just as well for this use case, and be both cheaper and faster.

[jordanthoms]

Optane is in between DRAM and flash in terms of cost (and performance) - it's also denser, so you can fit much more storage on each DIMM slot.