We love SotMK! You can get a pack of cards for free every time you enter MK, so if you have parkhoppers it's not out of the question to stop in multiple days just to grab your cards

We did that on our last trip.
If you've ever played a card based battle game (Pokemon, Magic, YuGiOh) then the benefit of buying cards will make sense. Different cards have different powers/strengths/spells so stronger cards "theoretically" give you a better advantage when battling tough bosses at the portals. But all cards are blind packs so you never really know what you're getting. I never saw the need to buy cards when we could build a deck just by visiting MK a few times.
The portals are positioned through the park, and you go on guided "missions" to defeat villains. Portals are unlocked by scanning the magic band of whoever you designate in your party to be the keyholder. You hold up cards to cast spells. The camera of the portal scans for the design on the outside of the cards to figure out what "spells" you are using and giving you an outcome. I'm not honestly sure if there's power calculations going on to determine success, I don't believe you can "fail" by using the "wrong" spell or a low powered card. But it's tons of fun