This makes them very unlikely to find your website or too find the service, whereas f they get home and find a business card in their back pocket then they'll recall the conversation and have all the details they need right there (and if you get plastic business cards printing or laminate business cards printing then it won't matter if they go through the wash once or twice first). At the same time, few people throw away business cards, particularly if you get plastic business cards printing, which means they can be re-discovered at the bottom of drawers or in wallets months later leading to repeat business.
You might think that this one extra person that you will be getting business from wouldn't make the largest amount of difference to your company. However this is a mistake, and successful businesses realise that every single client or customer is highly valuable and can conceivably lead to many more. If you satisfy this one customer for example, they then may go on to tell their friends and spread the word and then these people too could continue to spread the word further.