Completing the various missions will earn you different rewards which can includes cards. Doing the missions are wothwhile as it gives you an opportunity to earn cards and also gives you some direction. All missions are timed so you should consider prioritising those that are going to expire sooner or those that offer rewards that are useful for your park. Remember, once a mission expires and disappears you will no longer be able to get that missions rewards. You will find that many missions will require you to upgrade a card, place a number of cards and other card-related things.