This is a question we got recently from one of our clients, and it's a topic worth discussing.
The answer depends on how they are used after you get them.
Most business cards end up in a drawer and are used occasionally, which is how an excellent communication and targeting resource has been wasted.
We have a few marketing hacks, but you can add your ideas if you want, based on the specific of your industry:
1. Send a thank you note via email after each meeting and ask them to join your social media channels.
2. Add all the business card information in a document or CRM, including meeting notes, the probability of partnering in the future, etc. We recommend HubSpot.
3. Create and organize your contacts into audiences. You can start with your customers, prospects, partners, service providers, etc.
4. Keep your audience warm.
a) Craft different pieces of content for each type of audience and create an email cadence (once per week should be enough) that you stick with religiously. For example, your customers might be more interested in new features released or more futures on their existing plan for only X dollar extra. Your prospects might be curious about your company's growth and reputation, so consider sending your company achievements, new products or services releases, trade fairs or events, webinars, blog posts, etc.
b) Keep the message personal. You may reuse some content but add a bit of context in the introductory part.
5. If the database gets enough, say 1,000+ contacts, you may advertise your prospects directly or create look-alike audiences in Google Ads and social media.
6. The prospects that show interest in your content (emails open and clicks, organic posts, click on ads, visit your website, watch videos), should be reached by sales or yourself if you know the prospect personally.
The more touchpoints you create, the more likely you are to convert.
The short answer, they are highly targeted. You have acquired offline what it takes months and lots of money to achieve with paid campaigns.
Just a friendly note. Before starting, check if further email communication falls under GDPR and you need communication consent.