If you met two individuals and one was scrambling to find a pen and something to write on and the other person simply pulled out a business card, who would you want to do business with? Showing that you are prepared at all times is a great indicator that you are professional.
A business card is a physical object that a potential prospect leaves the encounter with. Your brand stays with them. If you meet a prospect and exchange email addresses and phone numbers you each walk away with another contact on your mobile phone -- it ends there. If you hand over a creative business card that makes a great impression that person is likely to show it to other people -- putting you and your brand in front of additional prospects.
You want your business cards to make an impression. They should have a creative design that makes your business stand out. There are a number of styles of creative business cards, that range the gamut from expressive to more minimal design. Your business card should match the brand of your business and be creatively aligned with your goals.
When you meet someone that could potentially be a great prospect or connection, do not you want him or her to walk away with a great first impression? A memorable business card does a lot more than just pass on an email address or phone number.