I remember one of my first jobs. I was working in the office of a production plant manufacturing tanks for carbon dioxide. The job was easy enough most days. I answered phones, filed papers, and even help produce the manuals to go along with the tanks.
Some days were a little slower, though. On those days, I printed out address labels from a long list, then sent out information on our company to the companies on the list. I used to wonder where they got all the addresses from, but I now realize that the lists were purchased from a company specializing in matching companies with addresses of potential customers.
Each Lists Company matches the company to potential customers based on a number of factors: location and probability of a potential customer needing the product offered, for example. Then the company purchasing the list is supplied with lists of fax numbers, addresses, or telephone numbers for their potential customers so that they can be contacted.
There's so much more to getting those addresses than I could've imagined back then.