"Leaving the gun in the car I understand, but why unload?"
Some jurisdictions (Denver, for example), require that the gun be unloaded in either a locked container or locked compartment in a locked car when unattended...variations on this theme exist from political enclave to political enclave, such as where you can stow the ammo.
This especially bothers me with autos, since even more manipulation is required to get that top round in the chamber for full capacity when you reload it because you have to pull the mag to top it off after chambering the first round, then reinsert it.
In addition, the act of closing the slide tends to push the bullet slightly deeper in the case with each reloading from the mag. (~0.002", more or less, with one gun of mine.)
Terry