Why did Johann Wolfgang von Goethe marry Christiane Vulpius in 1806?

Johann Wolfgang von Goethe formally married Christiane Vulpius in 1806 primarily to secure her position during a time of significant instability and danger.1

Following the Battle of Jena on October 14, 1806, Weimar was occupied and sacked by French troops. Christiane had demonstrated great courage in managing the soldiers who were billeted with the family during the occupation. By marrying her in the vestry of the court church five days after the battle, Goethe sought to provide her with formal legal status and protection amidst the chaos of the Napoleonic period.1

Prior to this, the couple had lived together for many years—having begun their relationship in 1788—but Goethe had previously refused to undergo a church ceremony, which meant their union could not be formally acknowledged by society.2