The reason why aircraft are tested at the Nevada Test and Training Range, is due to its large size and dry lake-bed which can serve as a long runway for take-off and landings. Its remote desert location is the perfect place for experimental aircraft, air to air combat exercises, and other critical needs the Air Force has, away from civilian population centers.
A region of Nellis Air Force Base Test Range called Groom Lake houses facilities for the United States Army, Air Force, Navy and the Central Intelligence Agency (CIA).
Area 51 started off as an Air Force Training Center in the 1950's, and it was the proving ground for some of the most cutting-edge experimental aircraft such as the U-2, A-12 OxCart (predecessor to the SR-71), and F-117 stealth fighter bomber.
Area 51 is a remote detached facility far from but administered by Edwards Air Force Base in Southern California.