I worked in a mens store some years ago. There was many theory's as to what compelled customers to make purchases, often of products they didn't really need. Seeing empty shelf space, or a half-empty rack, and "clearance signs"... was supposed to give the impression of selling out-of-stock and create a sense of urgency to buy. It costs Performance NOTHING to show that stock is selling out. And... it may promote a feeling of needing to buy what you want... while it's still available.