Well, that's it, isn't it? Fitting in.
It's an unfortunate fact of life that succeeding in social situations (including business ones) often involves some sort of conformity. Eating is something that has very strong psychological connotations, and eating in a way that's markedly different from others sends an unspoken message that "I'm not part of the group". It's difficult enough when you really can't eat as part of the group (say, you must eat gluten-free, or you're a vegetarian). Doing so as an act of defiance against your family's social norms, when you realize you're also going against the norms of your work colleagues, can hurt you, for reasons that the people you're eating with may not even be consciously aware of. They just get an uncomfortable sense that you're the gazelle in the herd that's acting funny, and attracting the lions.
In a perfect world, she really should be able to eat in any order she wants. But our world is not perfect, so she needs to balance the satisfaction she gets from defying the restrictions of her childhood versus the uneasiness she's causing with her colleagues.
And what better way to find a perfect world than to support those of us who buck tradition and inspire innocent change?
I like tradition. I'm a fan of conformity. Conformity isn't a bad thing, even though it's often used as a pejorative term. We conform in all sorts of ways, which is what a lot of etiquette is about after all.
We conform to the norms of queing, otherwise we'd be seen as rude. We conform to the norms of keeping certain body parts covered in public. We conform to the norms of greeting by shaking hands, and saying certain things.
If there was no conformity at all, we wouldn't know what was going on.
Eating courses in a certain order provides structure to a social interaction. We can all agree to skip starters if none of us are very hungry, but if one person orders dessert first, then the rest might be wondering how long lunch will take now - will she want a main course as well, or is she just not hungry? Has she already eaten? They might be keeping an eye on the clock and wondering if they'll be late back to the office. Maybe it puts the waitress out of order, and she takes longer to get the meals out because she's waiting for dessert eater to finish so she can bring the mains out together.
Eating 'out of order' just adds a layer of uncertainty to the meal, because she has deviated from the known script. Now people, instead of just being able to mindlessly follow the script and interact with each other pleasantly, are focusing on the script and why dessert eater is deviating from it.
And yes, telling your work colleagues that you eat your food in a certain order is an act of rebellion against your parents seems very childish to me. And more so, because the vast majority of people had the same rules as children, and grew up to find them perfectly reasonable guidelines as adults.
I don't think it takes 'gumption' to order dessert first in a restaurant.
