It is said that advertising is a form of teaching - and direct marketing is amongst other things a form of advertising.
People learn through repetition.
That is why advertising campaigns essentially repeat the same message many different ways. A good example is the Boddingtons beer campaign, the Tango campaign and The Economist campaign. In direct marketing, though, we want to get a response immediately but the repetition is within the message. Obviously you don't say the same thing in exactly the same way. The brochure may express it one way, the letter another.