Halliday & Matthiessen (1999: 520):
Conceptually, perhaps the simplest way of expanding a process is by elaborating on it: saying it over again (or something very like it, with repetition as the limiting case), or else exemplifying it, or clarifying it in some other way. The grammar represents this relationship symbolically in English by prosodic means: the same intonation pattern is repeated … But since this does not appear in writing, various purely written symbols are used instead, typically ie, eg and viz.