John Boardley at I Love Typography:

Decorative borders were employed to demarcate or divide books, chapters or sections and, from the last decades of the fifteenth century, were used at the beginning of books as openers or title-pages.

I’ve always loved these sorts of illustrations and whilst mostly unrelated, may have something to do with the violet-y sumire drapes happening on this site.

Of I Love Typography though, John recently launched a Patreon campaign which is criminally under-funded.