Evaporating dish is generally being used for measuring any quantity left after an evaporation measure is done. Essentially after a hydrated salt is heated the waters of hydration is removed in the salt thus lowering the mass of the substance. After cooling, it should be measured immediately because the salt can acquire water molecules in the air thus forming the hydrated form again. The longer you let the evaporating dish being exposed, the more water molecules will stick around the salt. If that happens, the experiment will fail to present precise data.