7, pp156-160) identifies four major uses: (i) commenting on and justifying the title of the work; (ii) commenting on the text of the work (the most canonical usage); (iii) claiming a relationship to the cited author (name dropping); (iv) signaling the genre of the work (romantic and gothic authors use more epigraphs, classical and realistic fewer).
s authors use them both to set the theme and to link their work to the existing body of literature.