Crystallisation is the gentle way to get a heat-sensitive dissolved solid back as pure, well-shaped crystals — the method you use when boiling to dryness would spoil the product.
If you keep heating past the saturation point, you drive off all the water, including the water of crystallisation. Instead of neat crystals you are left with a dry powder — and a delicate solid may even decompose. That is why you must stop the heat in time. If you only need the solid quickly and it is heat-stable, evaporation to dryness is fine — but for good crystals, cool.
The slower a saturated solution cools, the larger and purer the crystals grow, because the particles have time to arrange themselves into a regular pattern. Rapid cooling gives many tiny crystals instead.