It's a bit hard to give you a definitive answer on this as you have to get the actual intent of the legislator that drafted the law.
I can think of a couple of things here.
1. If it is on your person you are less likely to leave it behind in a holster or laying on the seat unsecured when you exit the vehicle for any reason.
2. It's harder to draw the weapon and use it if it is secured on your person in a holster than if you modify a holster for attatchemtnb to a car seat, door or dash for an ambush type of situation for anyone approaching your car.
As far as the locked glove box is concerned, think where most folks keep their registration and insurance info. If there is a gun in there the best thing is to advise the Officer about it and let them open the box and secure the weapon. Remember, they have no idea about you and have to be ready to assume the worst.
I'm reaching a bit here on these two simply because it's second guessing the legislators intent. Frankly your best be is to find who drafted the law and simply send a letter asking for an explanation. They may have gotten some kind of input from Prosecutors or Police and written it accordingly.
Approaching a vehicle on a stop, traffic or otherwise, is one of the most hazardous things any Law Enforcement Officer does during the performance of their job. They rarely have any information on the occupant(s) of the vehicle and are walking up blind to the situation. Ambush in this situation guarantees the first shots to the ambusher. All the Officer can do is approach and observe as best they can and narrow the shooting situation for the occupant as much as possible. Staying back off te drivers window, door is one of the main tactics to do this. It forces the ambusher to turn farther and gives the Officer the best chance to spot the weapon before it's fired. It's also a more awkward firing position for the driver and may spoil their aim.
Hope this answers some of your questions.