Being ordered to leave your car and continue on foot is something most of us do not anticipate. But that is what happened during the Camp fire, and the odds are high that it will happen again. Evacuation traffic can easily come to a standstill, and possibly leave you no option but to continue on foot out of harm's way.
Having a car or truck full of bags, loose items, water bottles and lots of other things can be fine as long as you do not leave your car or truck. Unfortunately, you can get into trouble if you need to abandon your vehicle quickly and one of those "things" is not a go-bag. A go-bag will consolidate your most critical supplies - your documents, medication, plans, etc.
While having a well stocked car or truck can be part of a plan, another part of that plan should be to have that one key go-bag packed and ready to go. So, should the unthinkable happen, you will continue to have your most important supplies with you.