There are lots of folk tales about why we have Daylight Saving Time, but a blog posting on Energy.gov says the genesis of the time-change was to save energy in buildings.
In 2007, the US added four weeks to Daylight Saving Time by changing it to start on the second Sunday in March and end on the first Sunday in November. DOE estimates the four extra weeks save about 0.5 percent in total electricity per day, or electricity savings of 1.3 billion kWh. These electricity savings generally occur during a three- to five-hour period in the evening.
“Germany was the first country to implement Daylight Saving Time in 1916 to conserve resources during World War I,” according to the DOE blog. “The US adopted Daylight Saving Time toward the end of World War I and then again during World War II, but between 1945 and 1966, there was no federal law regulating it. This led to confusion between states, and in 1966 Congress passed the Uniform Time Act to establish uniform dates for observing Daylight Saving Time.”
Hawaii and Arizona (excluding the Navajo Nation) along with the U.S. overseas territories of American Samoa, Guam, Puerto Rico, the Virgin Islands and the Northern Mariana Islands do not observe Daylight Saving Time.