These pulleys replace the factory pulleys with aluminium replacements. These are lighter, and have less rotational mass. This means less energy is wasted accelerating a heavy pulley up to speed, which means your car becomes more responsive with better acceleration.
In the case of the Skyline GTR, large diameter pulleys are available. These reduce cavitation within the water pump, the power steering pump, and prevent the bearings from failing in the alternator on engines which run at high RPM for extended periods of time. Ideal for use on cars with higher rev-limits than standard, or cars which are used on the track.