Why did Peter Paul Rubens become involved in diplomacy for the Spanish Habsburgs?

Peter Paul Rubens became involved in diplomacy for the Spanish Habsburgs primarily because his status as an internationally renowned artist allowed him to travel freely between royal courts, providing a perfect cover for discreet political negotiations.1

Following the expiration of the Twelve Years’ Truce in 1621, the widowed Infanta Isabella engaged Rubens as a confidential agent to pursue peace between Habsburg-controlled Flanders and the Dutch Republic. His reputation as the "painter of princes and the prince of painters" enabled him to meet with sovereigns and ministers under the guise of discussing art or sitting for portraits, while actually conducting sensitive state business.1

Later, Rubens embarked on a diplomatic mission to secure a peace treaty between England and Spain, viewing it as a necessary step toward resolving the conflict with the Dutch. Despite initial skepticism from King Philip IV regarding a painter’s involvement in diplomacy, Rubens successfully negotiated the peace treaty of 1630, eventually being appointed "secretary of the king’s privy council of the Netherlands" to elevate his standing at foreign courts.1