Imagine you are in a room of 100 strangers plucked at random from the entire U.S. population, and you are trying to start up a conversation about, say, baseball. How many of these folks, plucked from all walks of life, do you think would want or be able to engage in this conversation with you?
Now imagine another room of 100 strangers, placed together randomly except for their being sports fans. How many people in this room do you think you'd be able to engage in conversation about baseball? Maybe 30 people in the room only like basketball or hockey, and another 40 people have only a passing interest in baseball. This still leaves 30 people who like to talk about baseball, and might be happy to speak with you about it, for no other reason than their interest in the topic.
When you are applying your energy and resources in taking your marketing message from person to person, you want to be communicating with the audience most receptive to what you have to say. There's a world of sales prospects out there for whom your product or service represents a solution to a problem, a delightful luxury, or a very useful tool. If you can stack the room with 100 of these leads, you'll be overwhelmed with new clients.
Direct mail marketing cannot deliver you only the addresses of those people ready and eager to buy your special Widget X, but, like the room of sports fans, it can give the addresses of people who have bought other types of Widgets, the addresses of people with the means to buy Widget X, the people whose lifestyle would be made much easier by Widget X. Probably a good number of these prospects would be interested in Widget X.