I would think there are multiple benefits to it. First, it would definitely make it more rigid, which is important for a large flat piece of metal which is likely quite thin. Second, it probably cuts down on the amount of time needed to mill each case considerably. Downsides would be slightly more weight and I would also think that components wouldn't be packaged as efficiently as possible with those grooves in there.
I still feel like we're overlooking something important here. I like your comment about saving cost on processing, but I wonder why that single flat piece needs to be milled at all. I also would why the ridges are so detailed. Look at the right side of the design. If they just went with straight lines instead of curved lines it would be as affective for the previously stated reasons for having grooves and cheaper than having to go around the screw points. I wonder if it's not milled at all but is modeled. Could this be liquid metal? A mold would certainly explain why you'd want the ridges and why a designer might go to the trouble or making it somewhat decorative inside.