An X-wing simulator was a machine used by pilots to simulate flying an X-wing, usually for training and learning purposes. The simulator also has an astromech droid slot where pilots could plug their droids in, even though the machine is capable of simulating astromech instructions. By using their own astromechs, it allowed them to have a more realistic training experience.
The Skipchaser simulation was used during the Yuuzhan Vong War to show pilots what it was like to fight coralskippers.