Although a western art historian can probably say more, I don't think there's some large overarching philosophy behind using wealthy and aristocratic people as models in paintings; after all they were the ones commissioning most of the art, and besides, many times artists frequented their same social circles. It's not much of a stretch to expect artists to include friends and patrons in their artwork, especially if their representations are flattering. There are tons of different ways artists could included representations of friends and patrons in their paintings: explicitly, like this painting depicting Ludovico Sforza and his wife Beatrice of Este, the Duke and Duchess of Milan, kneeling in front of the Madonna with Child surrounded by the "Doctors of the Church;" or more subtle, like Raphael's inclusion of his own portrait along with his friend Giovanni Antonio Bazzi in his School of Athens (plus, a whole bunch of his friends have cameos in that panting).

In fact, what I can say with certainty that patrons definetely also appeared as models in non-religious art: in your above-mentioned Birth of Venus, the model is Simonetta Cattaneo, born into an aristocratic genoese family and wife of Marco Vespucci, son of a wealthy Tuscan merchant adventurer (and cousin of Amerigo Vespucci, the navigator and cartographer who proved that the West Indies were not Asia's eastern outskirts, as instead had been initially claimed by Columbus).