They're for finishing collections in the achievement section for people which didn't get all the cards in previous card promos. Finishing any collections nets you some points which go towards your General Ranking points.
They don't give you any gift though, it's only for finishing the collections.

You can fulfill the objective "Open a luck item" for the weekly HoP by purchasing one of these, which comes in handy since it'll never cost you more than what you get back from the objective.

Thank you!

Answered x