In the nineteenth century, they were commonly used to indicate prosody — breathing pauses and the intended rhythm of the sentence — which leads to comma placements that seem odd to modern eyes but would have been perfectly normal to a nineteenth-century reader. If you read any Jane Austen, George Eliot, or other Victorian writers, you'll see that sort of usage everywhere:

> "It is a truth universally acknowledged, that a single man in possession of a good fortune, must be in want of a wife."