Volatility is the measure of how readily a liquid fuel evaporates into vapour at ordinary temperatures.

The governing principle in a petrol engine is that a spark can only ignite fuel that is already in vapour form and thoroughly mixed with air — liquid droplets do not burn cleanly.

Gasoline is therefore blended to be volatile so that, once metered into the intake or cylinder, it vaporises rapidly and forms a combustible air-fuel mixture ready for the spark.

This rapid vaporisation is especially important on cold starts and for smooth, complete combustion across the operating range, which is the primary reason volatility matters.

Octane rating is a separate property describing resistance to knock, not how fast the fuel evaporates, so raising volatility does not directly raise the octane number.

Volatility by itself does not automatically reduce exhaust emissions, nor does it prevent corrosion of metal parts — those depend on additives and combustion control.

Hence, the answer is To ensure rapid vaporization for ignition.