From “Louise de Kéroualle, Charles II’s French mistress: a discussion with Susan Holloway Scott” on Catherine Delors’ Versailles and More blog:
In 17th century royal courts, the role of the king’s mistress was a prestigious one. These women had the king’s ear (among other things!) and with that confidence came a great deal of power. They often acted as the king’s unofficial hostess, receiving important politicians and international diplomats in their quarters so that the king might meet them on a more informal basis. They often became involved in political negotiations, and it was expected that they accept bribes of money and gifts for the use of their influence. They were rewarded with titles, lands, and wealth for their services, and their children by the king were ennobled. Away from the Court, ordinary Englishmen hated them as a drain on the king’s finances and clergymen denounced them as strumpets, but in London they were celebrities, and people crowded after their carriages for a glimpse of their famous beauty.
While being a royal mistress wasn’t a semi-official post at the English Court as it was in France, where Louis maintained maitresses en titre, these women still were important in a way that few others of their time were. Dozens of women passed through Charles’s bed, but the overwhelming majority of them are now faceless and forgotten. The royal mistresses were different. Seventeenth century women couldn’t serve in Parliament, the military or the church, or the diplomatic corps, or attend university. The only path to power for an ambitious woman of the time was through a man, whether as a wife or as a mistress.”