Simulation allows for safe testing of experimental code and settings, and can help you practice using your Ground Station without having to leave your desk.
Crashing simulated planes is a lot cheaper than crashing real ones!
Most users should choose the Software In The Loop (SITL) simulator (dev wiki) as it can simulate Copter, Plane or Rover without the need for any vehicle hardware, and can be run on Linux, Windows and Mac OSX. It is also the easiest form of simulation to set up.
Some users may wish to fly “virtually” with their hardware in the loop. This is possible with Simulation on Hardware