An app developer might use remarketing if they wanted to promote a paid version of their game to everyone who played the free version and reached level 10.

Remarking allows a company to attract people who have already been on your website or game that may or may not have made a purchase. In this case, they did not make a purchase, but they did use the product for a free. If they remarket and essentially, retarget their audience for the paid version they may be able to get them to purchase the item. Retargeting their marketing to have their level 10 customers purchase the paid version can in turn increase their returns.