Overlays save on time, labor, and disposal costs.
If the existing roof has only one layer, lays flat, and there aren't any problems with the roof deck, removal is not necessary. Regardless of whether you overlay or tear off, you should properly ventilate your attic. In almost all cases, continuous soffit and ridge vents will provide maximum cross ventilation. If a roof has more than one layer of roofing, the roof should be torn off. In most cases this is a code requirement.