Customers can be very indecisive. Most of the time they like to compare your products first with your competitors before clicking that purchase button. This is true as shown by a research for comparative pricing which discovered that 94% of online shoppers take time in finding the cheapest product, 36% of which spend an additional 30+ minutes of comparison shopping before arriving to an actual purchase decision. By including a countdown timer to your promotions, you help your customers decide immediately, thus closing that sale right there and then.