This controls whether the write cache on the underlying physical drives themselves, not the RAID controller cache, is enabled for this virtual drive. An admin cares about this specifically because enabling physical drive cache can boost write performance but risks data loss on a power failure if there's no battery/capacitor backup, unlike controller-level caching which is usually protected. Setting this to disable would turn off the physical disks' own write cache for this virtual drive, trading some write throughput for safer data durability.