I believe for performance and error correction features for spinning hard drives. Check out the tables and graphic in the overview section here[1]. Any write to a sector requires updating the checksum, which means the whole sector has to be read before the drive can complete the write. Since the sector is probably in the kernel disk cache already, the whole sector can be flushed to disk at once and the drive can compute the checksum on the fly (instead of flushing 1 byte, making the drive do a read of the sector, recompute the checksum, and then do a write).

1: https://en.wikipedia.org/wiki/Advanced_Format