Salar de Uyuni, Bolivia becomes a mirror when covered in shallow water
Salt flats are, as their name would suggest, very salty. Minerals run into salt flat basins from the nearby surroundings when it rains, leaving only the hardened minerals behind when the water has evaporated. The result of this is that the top layer of salt flats are entirely composed of hardened salt, primarily common table salt. Because of this, some salt flats are harvested for resources.
Salt crystals form on the surface of salt flats
Salt can be harvested from Salar de Uyuni
Salt flats are almost entirely devoid of plant and animal life. With such high concentrations of salt, plants are entirely incapable of living, let alone the extreme dryness of the dry seasons. With no plant life or water, there's no incentive for animals to live out in the flats, and so they don't. However, salt resistant plants can sometimes live on the near outside of a salt flat.
Humans have found a use for salt flats relatively recently, other than for resources. Because they are so extremely flat and expansive, they turn out to be ideal places to race. Whenever a land speed record is broken, it is broken on a salt flat. Extremely fast cars must be very low to the ground, so flats allow for these vehicles to drive without the interference that they would get elsewhere. And because nothing lives in these salt flats, a racer can have no worry of harming the environment (apart from chemical emissions). The Bonneville flats that I mentioned earlier are used extensively for racing and speed testing.
The fastest cars are the lowest to the ground.
Salt flats can be fragile, however. The crystals that form on the surface are delicate, so people and vehicles can make a huge impression. For this reason, some flats are protected by the U.S. government. Death Valley is one of those places, people are only allowed to walk or drive along specified paths.