Not an aviation guy, but a car guy. Lead has historically been used to raise the octane rating of gasoline. Octane rating expresses the resistance of the fuel

Not an aviation guy, but a car guy. Lead has historically been used to raise the octane rating of gasoline. Octane rating expresses the resistance of the fuel to "knock" which is the sudden explosion of the air-fuel mixture inside the cylinder versus a controlled burn and which can damage the engine. The main factor that raises the octane requirement of an engine is the compression ratio (max cylinder volume to min cylinder volume). High compression ratios generate more horsepower but require higher octane fuel. The other thing is that lead lubricates the valve seats preventing excess wear, but that's a solved problem with hardened valve seats.