Turns out those character cards you get do something after all! Upon unlocking all items in the Arcade's Toy Room (in all categories), you will unlock Wario Kard on the Arcade menu. It is a special card game that allows you to use the cards you collected to beat opponents and earn more coins. The Capsule machine used will also disappear, being replaced with two "Shuffler" machines. These will dispense random character cards, including "B" and "A" rank versions of cards in your collection.