Plastic roof cement should never be applied to the bottom edge of a shingle to seal it because?

The application of plastic roof cement to the bottom edge of a shingle can lead to water being trapped. When sealants like plastic roof cement are used improperly, they can create a barrier that prevents water from draining off the roof effectively. Instead of allowing moisture to flow away, the cement can hold water against the shingle, which can contribute to premature deterioration or mold growth. Proper installation requires ensuring that water can easily flow off the roof, maintaining the effectiveness of the shingles and preventing potential water damage.