Low-ish carb is great. It helps you endure long runs overall and it's a good habit in general. Especially for recovering after the race and getting all those calories back.
During the long run itself however, glucose packets are absolutely necessary. You run out of glycogen stores every 45 minutes or so. That's when you start cramping up and having trouble. A glucose packet gives you some caffeine (helps with stamina and pain tolerance), and some electrolytes (helps with cramping and hydration), and some 100 calories of sugar (helps with glycogen levels and mental fortitude).
Crucially, you aren't replacing all the calories you burn. That's impossible. In a 4 hour marathon I burn between 2500 and 3000 calories. The packets replace only 500 calories.
The difference between packets and no packets is that with packets, I am able to be a human person after a marathon. Far less cranky, far less tired, generally feeling okay. Without the packets, I'm basically a walking zombie.
That said, it took a lot of practice to figure out the right timing for those packets, to train my body to absorb them and so on. It's definitely not something you want to start experimenting with on race day. That's a recipe for puking.