You can create a game badge up to five times, by using each set as a whole. For each time of creating or upgrading your badge, you get 100XP, a random emoji and a profile background from that game. Additionally, you have the opportunity to get a discount coupon to use in the Steam Store.
Why to improve your badge?
You will receive a Mystery Card instead of a coupon during Steam sales, such as the Winter or Summer Sale. These can be used to make the sale badge.