The pump is submerged in the oil reservoir of a hydraulic power unit. I assume it's aluminum for production efficiency(they can order a long bar of that extruded profile and cut it into pump-sized lengths, and it's easier to machine), but I really don't know. These pumps charge an accumulator to 3000psi and unload until the pressure drops to the setpoint. The benefit is reduced energy consumption and a lot less heat being put into the oil, but the drawbacks outweigh the benefits IMO. I've seen two 5-gallon accumulators develop hairline cracks from the cyclical nature of these pumps. That's a 9/16" wall steel pressure vessel. Aluminum doesn't stand a chance!