To be closer to water resources. Most settlers In the early days settled in places where a water resource was near by. If there's water that means that there will be food to eat like fish and other animals that live in the water and come to drink the water. Water areas naturally are surrounded by resources that settlers need.
The Spanish built missions along the coast for access to key resources like water and food, enhanced transportation, and improved defense. This coastal strategy also facilitated gradual exploration into the interior of California. Starting with the first mission in San Diego in 1769, this pattern led to a network of missions stretching to Sonoma.
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