The Mission provides the Spanish with Faith, Science, and an incentive to expand to other continents. Found as many cities as possible on continents far from your Capital Capital, then build Campuses in them, surround them with Missions, and enjoy the rich spiritual and scientific benefits! As of Gathering Storm, adjacent Holy Sites to a Mission gives a +1 Science boost.
According to the Catholic Church, missions are religious communities devoted to teaching, tending, and helping the local populace, thus supporting Christ’s ideals. Of course, the Spanish, Portuguese and French missions were also devoted to converting the locals and then “civilizing” them through cultural assimilation. The Spanish missions serve as the traditional template, self-supporting enclaves with churches, dormitories, fields and pastures, gardens, barns, workrooms, hospitals and schools. In places these were even fortified, ready to defend themselves from opposition. Most of the missions in the Americas, Asia, and Africa were administered by religious orders such as the Franciscans, Jesuits, Dominicans, and Augustinians rather than the Church itself, but the effects were the same. Nonetheless, the missions often served as a nucleus around which European colonizing efforts could coalesce.