A card is included to show you which hiragana you can use with the modifiers. When playing the Moekana game, you can now make words that are modified by a dakuten, handakuten or chouonpu. For example Ka (か) becomes Ga (が), Sa (さ) becomes Za (ざ) and so on.
Once you have gotten a grip of the basic set of hiragana, you will then want to learn the dakuten and handakuten hiragana too. Just like any booster pack, you dont *have* to have these cards but if you do then they will aid your Japanese learning experience.