Collections are a way to upgrade your slot machines. You can get the items required by asking your friends, purchasing them from the App Store, or winning them from spins that include a Treasure Spot. When you collect enough, you can redeem them to raise the level of a slot machine.
Upgrading a slot machine has these effects:
1. A Scatter Win gives you one additional bonus spin per upgrade level
2. The payout for a Jackpot Win goes up
3. The starting Bonus Pool amount goes up. This Bonus Pool affects the payout when you collect all 5 bonus symbols within 12 hours.
4. You get Bonus Coins added to your account immediately when you redeem the collection.
If you open up a collection page, you can see the symbols you will need to complete it. The top row of symbols shown are unique to that machine, but the symbols in the bottom row are shared among all your collections. On the left, you can see what you will get when you upgrade: the new Jackpot multiplier, the new number of Spins in a Scatter Win, and at the bottom, the amount of bonus coins you will get immediately.