The goal of the cardgame is to break up the normal gameplay loop and obviously to get some rewards in the form of office decorations. Next update is going to add wagers to the existing challengers that you'll unlock once you've beaten all 3 of them.
Wagers unlock at different points per character depending when they'd be fine to do them. Snape is tied to your friendship lvl with him (drink in the evening.) Hermione's corruption level and the twins will do them off the bat.
Beating the wagers is another way to earn certain things in the game if you preffer doing it by playing the cardgame rather than normal method. Snape gives you a random potion ingredient/potion base. The twins will give you a percentage of their profits and Hermione... well, you'll have to find out for yourself once the update is out.
I posted a teaser trailer about it here a while ago on where I'm going with the cardgame
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