It's for the extended cabs. The extended cab hangs below the rails. This gets rid of the nipples that hang down from the carrier bearing. Here is what it looks like installed.
Ah, ok I see now. My 98 has the crossmember that hangs below the frame, my sons 96 is flush. I thought what you were saying is it raised the actual position of the carrier bearing for driveline alignment for lowered trucks.