He's interested in your work and stays in touch with you, perhaps sending an article about a related topic after you've talked. This initial contact might prompt you to one day call your newfound friend for advice, or to visit his company's booth at the next trade show and introduce yourself. Maybe it never goes beyond that, or maybe one day you'll work together. The point is, after a time you become known entities to one another. You become members of one another's circle.
People trust other people they know.
This approach to business reveals integrity and it reveals patience. People who have made that kind of investment in you are usually worthy of your trust. People in your circle are people you would hire or go to work for. If there's a catch to this, it's that you have to invest in getting to know them first. That's what makes a really good network of people so difficult to become a part of.