A purse would be left in the car if the owner of the purse wanted others to think foul play had occured.
That's what I thought, too, EXCEPT, Casey screwed the whole thing up when she called AL to pick her up. In order for this to work and the police to think Caylee and/or she were kidnapped (preferably by whoever stole the car) - there could be no witnesses as to where she was after she abandoned the car. So she would have needed to abandon the car and leave town that day in secrecy, leaving all her stuff behind so it would look like a real disappearance.
So how do we account for her calling AL to pick her up and her giving him that detailed story about how her dad was going to pick it up and how she was going to get a NEW CAR?
I'm not saying that the idea is wrong, I'm just trying to work out what exactly was going on.
And then, remember this, Casey told AL that her dad was going to get her a new car.