When you make a purchase of 1100 Gran or more from the Cashier tab, along with your Gran your account will be upgraded with Premium Features at no additional cost. The Premium Features include double the number of slots you have for Card Files under the FILE menu from 10 to 20. They also include extra space in your in-game mail(double) and additional theater functionality, which allows you to watch Replays of your battles, and share them.
If you re-activate Premium Features on an account that previously had Premium Features in the past, you will have all of your Card Files and saved Replays restored to you, provided it has been one month or less. Any more, and it is possible some of the data will be overwritten. If you purchased extra Replay slots, those will be restored to your account no matter how much time has past.