The turbo compresses air, and compression heats it up: at the compressor outlet, the air can exceed 150 °C. It is then less dense, so it contains less oxygen, and it causes knocking - which the ECU corrects by retarding the ignition, meaning less power. The intercooler cools it before it enters the engine.

The benefit is therefore not so much at peak power as in consistency: a saturated stock intercooler lets power collapse on the second lap of a circuit or during the third acceleration. Beware of the instinct to go bigger: excessive volume takes longer to fill and blunts response. The right intercooler keeps temperatures under control throughout real-world use.