As said above, it'll save your car and time, and if you save your ghost lap you have a few more options.
You can rename your setup in your settings (A,B,C) to the track that you set it up for. Usually I set a car up for a short track/low downforce, high downforce, and rough track (like rally or Nurburgring). So if your testing cars, this is an easy way to make sure you keep a certain setup.