Something running out automatically becomes more desirable.
"Only two rooms left at this price." Phrases like this create a sense of urgency that pushes us to decide quickly so we don't miss the opportunity. We perceive a resource as more valuable when it's less available.
Booking.com shows the exact number of rooms left at a deal price. eBay sends emails warning that a product is about to sell out, as a last chance to buy it. In both cases, limited availability, real or perceived, is what triggers the purchase decision, even though the price and the product haven't changed.
Limited availability creates a sense of potential loss. Without conscious thought, we tend to reason "if something is scarce it's because everyone wants it, and if everyone wants it, it must be good."
Source: Mittone, L. & Savadori, L. (2009). The Scarcity Bias. Applied Psychology, 58(3), 453-468.
Limited spots in a training program, slots for a social benefit, or units of a product only work if the scarcity is real. Using it falsely erodes trust faster than it gains in conversion.
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