Defrag on Large HW RAID-5 Volumes

What benefit would defragging a 100+GB volume that is stored on an external hardware managed RAID-5 disk system that is online 24/7/365?

I understand how defrag would help single physical disks and RAID-1 configurations, but how could it possiblyhelp multiple drive/head configuraitons, since this is invisible to the O/S.