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.