This feature of the game allows you to automatically stock up on EXP over time, regardless if you’re online or offline. The amount of EXP that will be stocked can be improved by clearing the floors of the Tower of Decadence (Daily Dungeon). If you purchased the Monthly Card, you’ll be able to collect 30% more EXP. However, it's not really that much of a deal, especially if you're accessing the game regularly - enough to collect the accumulated EXP before it gets completely filled up.