Goal Gradient Effect: The tendency to approach a goal increases with proximity to the goal.
Article: Goal Gradient Effect: How rewards can improve your customer experience
A 10-space coffee card pre-stamped twice will motivate the customer to complete the card more than an 8 with no pre-stamps. As proposed by the American psychologist Clark Hull, the Goal Gradient Effect describes that people are motivated by how much is left to reach their target. In other words, as humans are getting closer to a goal, the motivation to make efforts toward that goal increases.
The application of this effect can be easily found in many loyalty programs (Starbucks' loyalty cards) or the use of progress bars (LinkedIn's profile completion process).
This effect also reminds me of the Zeigarnik Effect, which I learn at Weekly I/O #27, that states people remember uncompleted tasks better than completed tasks.