Design can never be an ultimate explanation for anything. It can only be a proximate explanation. A plane or a car is explained by a designer but that's because the designer himself, the engineer, is explained by natural selection.
Dialogue must appear realistic without being so. Actual realism-the lifting, as it were, of passages from a stenographer's take-down of a 'real life' conversation-would be disruptive. Of what? Of the illusion of the novel. In 'real life' everything is diluted; in the novel everything is condensed.