You can get a certain number of cards at a point (starting with 3 going up to 6) and you can’t use a card if the shelve is full. So for example you have the bread shelve full and you get a bread card, you won’t be able to use it. Let’s say you need a cashier card at that point. In order to be able to get a new card you need to trash a card. Considering that you have the maximum amount of cards you can have.
It’s not a very important thing, but without it you could lose.