Answer:
This was done in order to colonize the area, then known as Alta California.
Step-by-step explanation:
While the Spanish had discovered California over two hundred years ago, it was mostly a wilderness with very little Spanish presence.
With a large Empire to manage in South America, it was never really given importance. However, by the 1700s, the Russians in Alaska were becoming ever more adventurous, sailing down to California's coastline.
In order to secure the area, the Spanish had to make permanent settlements. Similar to how they colonized other parts of the world, they called in Franciscan missionaries to build their own missions across the California coastline.
This would also encourage other setters to move into these areas and eventually populate them.