What is the significance of performing mouth-to-mouth breaths during CPR?

Exhaled air contains a higher percentage of CO2 than inhaled air, due to it being a waste product of respiration. By breathing excess CO2 into the patient, the blood acidity will decrease and will be detected by the pH receptors in the hypothalamus. This will stimulate the respiratory muscles- the outer intercostal muscles and the diaphragm will contract, causing the chest cavity to be pulled upwards and outwards, decreasing the pressure within the thorax and allowing air to flow inwards. This should help to restore normal breathing in the patient.