What is Daylight Saving Time and why does most of Arizona not observe it?

According to the Congressional Research Service, the states and U.S. territories who do not observe daylight saving time include American Samoa, most of Arizona, Guam, Hawaii, the Northern Mariana Islands, Puerto Rico, and the Virgin Islands. 

Daylight saving time in the United States was established by Congress under the Standard Time Act of 1918 as a "way of conserving fuel needed for war industries and of extending the working day," according to the Library of Congress. It was repealed following the end of World War I, however the issue reemerged during World War II and Congress established it yet again in 1942. In 1966, Congress established the Uniform Time Act to clarify issues surrounding the consistency with time observances under daylight saving. The rule allowed a state to exempt itself, or parts of the state that lie within a different time zone, from DST observance. In addition, the law authorized the Department of Transportation to regulate standard time zone boundaries and DST