It's the write cache in a storage system. If you're using these servers to build a scale-out storage system, hyper-converged or not, you can receive new data, write it to the NVDIMM, replicate to the NVDIMM in a second server and ACK. There are APIs you can use to ensure the data's been flushed from CPU cache to the NVDIM, and is therefore safe from power loss. Total time 250us or so.
If you only have standard DIMMs you can't ACK until the data is someplace safe like a flash drive. Writing data to an SSD, even an NVMe SSD, will push overall latency up to 1-2MS.