It’s a test facility for the United States Air Force (USAF).
When the USAF initially purchased the site in the 1950s, the military used it to test aircraft, including the U-2 espionage plane.
This is a high-altitude reconnaissance aircraft that can fly in the stratosphere at altitudes greater than 70,000 feet.
It’s also not surprising that Area 51 has become a hotbed for reported sightings of unexplained flying objects, considering how secretive the Air Force installation is.
Even the U-2 was expressly mentioned in documents made public after a Freedom of Information Act request in 2013.