Handle special case where Customer has multiple cards, and we want to charge a specific card

From stripe documentation -  https://stripe.com/docs/api/go#create_charge
>A card to be charged. **If you also pass a customer ID, the card must be the ID of a card belonging to the customer.** Otherwise, if you do not pass a customer ID, the card you provide must either be a token.

The case where both CustomerId and Card is present, card should be the ID of one of the cards of that customer. This is useful for the case where there are multiple cards and you want to charge a specific card of that customer.
why store this in metadata?