That's not the whole story obviously: it's thinner at the edge, but is pretty much the same thickness at the center, which isn't buying you anything for a footprint savings if desktop space is at a premium (which for most, it probably isn't).
I'd of rather seen Apple just reduce the thickness of the machine across its entire length, and retain the SD slot on the side (or maybe consider putting the slot on the bottom, along with the USB connections). Kind of like how they thinned down from the cMBP to the rMBP: no optical illusions going on; just thinner across the board.
Someone said that doing it this way let the metal "back" of the thing act like a better heat-sink. I don't know if it's true, but it's one of those things I could probably "buy"
I guess if the overall unit is lighter that helps them with shipping and all of that. Shipping thousands and thousands of units, even a few ounces adds up.
Beyond that though it's just for "aesthetics" so long as you're not looking at it from too sharp of an angle.