I keep a loaded handgun in "my truck".
That said, the truck I speak of is my issued patrol vehicle for the agency I work for. The pistol is in a "go bag" in my back floorboard, along with a spare holster, mags, knife, duct tape, para-cord and a flashlight The reason(s) is/are as follows:
Sometimes I'm called out late at night, and in a moment of "haze", it's possible to walk out of the house and try to do "cop stuff" and forget my gun.
I have also had my pistol locked in a lock box in our jail, and ran out of the place to go to a call and found I had forget my pistol in the lock box. In both of those cases, a gun in my vehicle has proved very useful.
I DO NOT leave guns in my personal cars, period.