I find business cards hugely useful as the client. If you don't give me one, I'll forget how to contact you in a few hours.

I have a few inch tall stack of business cards I've collected over the years, mostly from people who wanted to sell me or the company I worked for at the time something. When I or my company want to buy something, I go to that stack and can quickly find contact info to reach out to one of the people and get quotes or info.