"Calling cards became popular at the end of the 18th century and bore the visitor's name, title and residence." (Grace, Maria. "Morning Calls and Formal Visits: Socializing in the Regency Era" English Historical Fiction Authors, http://englishhistoryauthors.blogspot.com/2013/08/morning-calls-and-formal-visits.html . Accessed 24 April 2017.) The purpose of the calling card that Willoughby leaves behind at Mrs. Jennings' residence serves to inform her, as the mistress of the house, of his presence in London. As her acquaintance, it would have been considered rude of him not to do so.