Some groups like T1 diabetics can not have the amount of sugar in "normal" fizzy drinks. That amount of sugar can cause issues from nausea to messed up glucose levels needing hospitalization. Hamburgers and fries are not a problem (not healthy but not dangerous).
"Diet Coke" is just a stupid name for sugar free drinks.
Other advantages: no need to brush you teeth immediately after drinking soda to prevent cavities, lighter mouth feel, safer in a car (if you spill the drink there's no sticky residue), T2 diabetes risk reduction vs drinking regular soda, no added insulin response, and if you drink a lot of liquids you can have a few refills without thinking about calories.
Artificial sweeteners also increase appetite and cravings for sweet food, so if you are suffering food-related illness, it's better to stay away from them as well.
Not an expert, but I would imagine the speed at which these carbohydrates get metabolized is a factor in whether or not it messes them up? Carbs from bread take a much longer time to raise blood glucose levels and do so for longer than a spike induced by drinking what is essentially liquid sugar.
Drinking sugary liquid like coke causes very rapid rise in blood glucose levels and will require excessive amounts of injected insulin to maintain glucose in reasonable limits (which may lead to dangerous drop later).
Eating something with high carbs is much slower and can be managed with normal insulin doses. White bread is not the healthiest diet, but a burger and fries every now and then is not a problem for a T1 diabetic whose treatment is under control.
Like — why are you buying this totally unhealthy meal and then getting a diet coke? Who are you kidding?
Yet — I’ve been that guy. So it’s not so mind boggling.