you can go whenever you want, she gives you 3 cards/each map you have 100% explored.
about boss card, for now as far as I know they are used by sorcs, necro, (devil type) wugushi (insect type) and druid (plant type?). i didn’t try any of those classes tho. you can use cards to upgrade gems also. some cards are required for collections
some people use the mini game of card battle to trade or ‘sell’ cards. however, because more game content will be added in the future you might want to keep them as you never know for what you might need them. also there is a high change other classes that will make use of cards will be added to the game (eg. fortune teller)