By "wherefore?" Juliet means "for what purpose?" So think on this. For what purpose is your business card? Ah-ha! Yes, it's a means to network with others. It's a means to remind that person exactly what you do and how you both can work together for mutual benefit.
I just had to post about business cards this month. This past weekend I had the pleasure of attending Katsucon 15 (an anime convention with over 6,000 people) and being an alternative business person, I was invited to a VIP party. I was amazed about how many of the other like-businessed folks had either run out of business cards or didn't have any on their person whatsoever. Don't make this mistake! Yes, you're going to the party for fun, but you should also be going for business. For networking. For success.
Don't ever run out of business cards. Ever. When you're inebriated, it's easy to spell your website or phone number wrong. At the end of the night, that bunched up napkin you used to scribble your company name and website might look like a bunched up napkin (instead of your impromptu business card) to the person you gave it to. What happens to bunched up napkins? They become nose wipes and then trash. Or just trash. The point is, why take that risk? It's easier to mistake a napkin for trash than a business card. You could have missed out on that one, ultimate, break-through opportunity because of something that could have been prevented.