> Couldn't you just write data and call sync?
No. One of the reasons is that the sync does not propagate
down. Devices with own caches have no way to get this information
and use it to flush their caches.
Additionally you don't always want all and everything flushed.
Some pieces of data always need to be on disk, others can be written
delayed or even out of order. Those are patterns that you can't
implement without RAW devices.
Mike