It is probably to stop drafts and keep the heat in. On my CubeX, when printing with open sides it is very susceptible to temperature changes. It would cause peeling up of the bottom layers. By enclosing the sides with panels, the heat of the electronics + extruder is enough to keep the inside toasty and warm and mitigates this problem.

I agree that this is a very nice looking machine. The XY/spline drive is a great idea with a lot of advantages. It will be interesting to watch the progress of this one and the M3D Micro, as both were on Kickstarter at about the same time.