Synchronous transfer is simply faster, and there's Ultra SCSI beyond that, too. ;-) (plus U2W, then U160, U320 - sigh...)
My old A3000 can do ~8 MB/s with a 15 year old 1 GB Toshiba, that's pretty much all you get out of async SCSI. 10kRPM drives can easily do 40 MB/s.
It also speeds up 'simultaneous' I/O transfers on several disks because it reduces bus load.