To keep the temperature from rising too high, and to allow solvent vapors to escape. Paints continue to release solvent until fully cured, and some (particularly if heavy coats have been applied) will continue to do so for a time after full cure. Venting will also prevent hot spots from forming inside the box.
Cutting a number of small vent holes in sides at the top and bottom of the box will do it.
Also, allow the model to cool to room temperature before you handle it too much. Warm paint is soft paint.