You then offer your postcards for free and allow people to take them when they are at trade shows, open evenings, launch parties or just in your waiting room. This is a gift for them as it means they don't have to pay for cards to send to their family. This then makes the company look good and generous in the eyes of the recipient, but in fact it benefits the business much more.
Here then the person will look at the card for a while in their own home, before sending it to friends and family who will then get your message and you have immediately extended the reach of your marketing message by having it sent to that person's network of friends and family.
Alternatively a business may just use postcards printing in order to send messages to their clients and staff. This can be once again a great form of promotion and can help them to spread their message into people's homes and other company's offices, but it can also be a very nice to send a gesture of thanks to clients and customers who have supported you or other companies you have worked with.