From The Telegraph:
Diana, Princess of Wales, was lucky to have only three people in her marriage; King Solomon needed 300 mistresses to keep him happy. Married life might be less crowded today than it was at the start of the previous millennium, but the mistress has by no means disappeared. In Mistresses, Elizabeth Abbott looks at 80 'other women’, from Greek concubines to Camilla Parker Bowles, and asks why they chose to live in the margins of someone else’s life. We all know what men get out of the deal, but what’s in it for the mistress?
The answer is usually money. Financial support is 'integral to mistressdom, indeed one of its most attractive features’. The late Sir James Goldsmith observed that a man who marries his mistress creates a vacancy: mistressdom is a profession, replete with perks, promotions and – all being well – a pension. For 40 years, Marion Davies was swathed in mink and doused in champagne by the newspaper mogul William Randolph Hearst, who kept his wife on the east coast and his mistress on the west coast. 'May I be a mother to you?’ he asked Marion. She would have preferred him to be a husband, but settled for calling him 'daddy’ instead.