Why is anhydrous sodium sulfate used in organic chemistry labs?

Because of its affinity for water, anhydrous sodium sulfate is often used to dry hydrophobic solvents used in aqueous organic extractions. For example a water based sample is extracted for organic compounds using dichloromethane, or ethyl ether in a separatory funnel. The organic layer is passed through a buchner funnel holding anhydrous sodium sulfate, yielding dried solvent extract.