Once you buy the Mystery Pass as part of every Mystery Event, you get a couple of special perks! You will receive 5 Gems every single day for the duration of the Mystery Pass after purchasing it. You will also get some unique decorations or a new pet, and five additional inventory slots. Keep in mind that these extra inventory slots are going to be available only for the duration of the Mystery Pass and the items that you have placed inside them will automatically be returned to your Pocket once the Event ends.