Business cards give room for experiments with materials and technologies;
Thanks to the size, they are convenient to carry.
A business card is a very personal thing: it traditionally bears the name of the owner.
In addition, it encourages to action, because it must be given away - this is the business card's purpose.
In this case the keys are not just business cards, they can be given only to those people and in those situations that are consonant with the key.
These business cards direct the attention of their owner to the problem - and "prompt" who will help to solve it.