Why did some Americans think that making California part of the us would strengthen the security of the nation?

Owning California provided a long coast line on the west coast of the continent. If it belonged to someone else, they would be in a good position to push eastward and control the land at least up to the Rocky Mountains. Control provided protection and a built in defense against someone invading from the west. And the resources available in the form of timber, gold, silver and fertile areas were important to a growing nation.