Why would the government concern itself with building a dam in the middle of the desert?

The desert is an ecosystem that lacks the necessary rainfall in order for populations to effective use of water as a resource on a continuous basis throughout the year. For example, the coastal part of much of the state of California is characterized for being a desert. However, these areas are densely populated. As a solution to this issue, civil engineering has created Dams, which are walls of concrete that retain the flow of water from rivers located many kilometers away from the coast. These walls retain water that would later be released by the operators of the dam in a gradual way to distribute the providing of water equally throughout the year. There are between 1300 and 1400 of these structures in the state of California.