A battery-backed write cache prevents pending writes to HDD-type devices being lost when power is lost.
To improve performance, most modern OSs' drivers don't wait for confirmation that a write has gone all the way through to the platter on hard disc drive devices before confirming write success to the higher layers of the OS, and application. If power is lost to a drive array while writes are still pending to spinning media, an inconsistent file system may result, leading to data corruption and/or loss.
Putting battery backup onto any write cache in the pipeline helps ensure that queued writes are not lost when power is lost. Part of the bootstrap procedure for such disc arrays involves replaying the contents of the cache to get those out to spinning media before the array is available for mounting.
