The unlocking in the hopes of preventing a window smash.
Reminds me of a joke.
I leave my car with a sign hanging from the mirror that clearly states that no valuables are inside.
One day I go to my car with a broken window and a new sign that says "just checking".
What I would be worried about when leaving my car open is eg other people sleeping in it. (I wouldn't mind too much, as long as they are clean. But that's too much to ask for.)
Fortunately, I do not own a car in the first place.
Leaving the car open is mostly to forestall people opportunistically smashing the window and looking for valuables.
Fencing a whole car is much harder than fencing some smaller loot.