It allows the laminate in the boxes to "become one" with the house environment (moisture levels and temperature). If you do this before giving the flooring a chance to do this, you could end up with gaps and/or bulges from the shrinking and swelling of the planks.
I suppose you could accelerate the process a day or so if you were to open the boxes and separate the planks out so they aren't all one large stack (maybe 5 or so to a stack). That'll help the temperature stabilize more quickly. However, doing anything but exactly what the instructions tell you may void any warranty from the manufacturer, so take my suggestion at your own risk.