You may be missing the purpose of NVRAM(Non-volatile or battery-backed-up ram) on a RAID card. The idea is during a write, the controller cannot correctly acknowledge the write until the data is committed to durable device like disk. This vastly slows down writing to disk. With NVRAM, as soon as the data is copied to NVRAM, you may acknowledge the write. Software raid cannot do this unless it has access to NVRAM. When power is restored, the writes still in NVRAM are committed to disk. This is critical for high-speed databases to maintain consistency.