If you want to know why many promotions use a code that you must look up online, this is one of the reasons why. It allows the company to control after the fact how many prizes are given out.

Previously:

https://news.ycombinator.com/item?id=33306801

https://news.ycombinator.com/item?id=20667774

https://news.ycombinator.com/item?id=15993056

https://news.ycombinator.com/item?id=6861677