Why does salt lower the boiling point of water?

Actually, salt raises the boiling point of water, not lowers it. This is called boiling point elevation, a colligative property. When you dissolve salt in water, the sodium and chloride ions disperse throughout the liquid and interfere with water molecules escaping into vapor. Because more energy is needed to overcome this interference, the solution requires a higher temperature to boil than pure water does. The effect is real but modest. Typical cooking amounts of salt raise the boiling point by only a fraction of a degree Celsius, so the practical cooking benefit is negligible. The real reason to salt pasta water, for example, is flavor, not meaningfully faster or hotter boiling.