Final Polishing: It catches any tiny carbon "fines" (black dust) that might have escaped from the GAC filter.Extended Contact Time: Because the water must travel through a solid block rather than loose grains, it has more contact time with the carbon, leading to superior removal of chlorine, VOCs (volatile organic compounds), and heavy metals.Dual Filtration: It acts as both a chemical filter and a fine sediment filter, usually catching particles down to 5 microns.