This OID maps each volume to the RAID group (array) it belongs to, so you know which physical array backs each logical volume. You use this for troubleshooting—if a RAID array is degraded, you can immediately see which applications are affected by checking which volumes sit on that array. For example, 'LU 1: 2; LU 2: 2; LU 3: 1' shows Volumes 1 and 2 both use Array 2 (and will both slow down if Array 2 is rebuilding), while Volume 3 uses Array 1.