Why include scenes of decadence in a book about a Christian doctor? A novel needs an arc provided by the plot, and in a historical novel, the plot must be driven by the realities of the times.
In 1918 the Germans lost the war, and the Treaty of Versailles humiliated them and made economic recovery harder than it might have been. During the 1920s, one reaction to the loss and hard times was a surrender to pleasures, including inebriation, drug use, partying with music and dancing, sexual escapades and sexual performances both literal, artistic, literary, and cinematic. Even the victorious allies suffered this malaise of material and social liberalism; in the United States, the period was called “The Roaring Twenties.”
Luther, the protagonist, encounters these times when he goes away to the university. He gets intoxicated for the first time and loses his virginity with a free-spirited English student, who lends him some of her erotic art books and sexually explicit novels. Then he has an affair with a prostitute. Later he gets entwined with a young libertine noble (junker) who leads him into doing cocaine and participating in an orgy.
Nonetheless, middle-class Germans were conservative and hoped to find a way to control the promiscuity and licentiousness that was occurring in the cities of Germany. Luther tries to understand what is happening and ultimately rejects the open lifestyle and returns to his middle-class roots.
The party ended, however, with the stock market crash in 1929 and the subsequent Great Depression of the 1930s. Economically, the Germans suffered more than most countries, resented the burdens of war repayments, yearned for order, and were willing to listen to anyone who could give them hope. Thus, the country became open to radical solutions like Nazism offered.
Therefore, the decadence in the book is both a reflection of the times and sets up Germany and Luther’s life for what comes next.