Butterflies are the premium currency here, and they'll help you get through difficult levels or earn more awkward prizes. You can even earn a singular butterfly in the shop, simply by watching an ad - though it's not like a single butterfly is worth much... Stock up on them as you play and you'll be able to purchase Special Boxes in the shop, or maybe you'll just use them on more ladybug currency of level revives. Regardless of how you spend them, they're handy to stock up on.