A few reasons...
Firstly, whenever I eat a lot of sugary foods, I get pimples. Usually I have fairly clear skin, but when I OD on junk processed chocolates (ie, around Easter Time) I break out. All over. Chest, shoulders, face. It's not acne standard, we're talking 3-4 at a time, but it's still very very annoying. Especially when you went and got fake nails the other week and can't pop the damn pimples because crazy thick nails are next to useless for all the things you need them to do.
Secondly, when I eat processed crapola like too much bread, or (non eggplant crust) pizza, I start to get fat in a really odd spot. Well you know, as well as in the usual spots that fat loves to hang out. It's my neck. I get a fat neck. Like my chin would like to be much better friends with my chest, or multiply, or something. But this is a particular "I'm only doing this for processed carbs" spot. It doesn't matter how thin I am on the rest of my body, if I'm eating wrong my neck stays flabby. And if I put on weight eating too much GOOD stuff, I just get a curvy butt but my neck stays slender. Bizarre. It runs in the family too, but my other siblings are pizza/pasta addicts and refuse to experiment with giving them up.
Lastly, alcohol. Alcohol is shit. It dries me out so badly, no matter how much water I drink to balance it out. And it makes me gain fat in a specific spot too. Which is my midsection. If I'm super healthy in all other ways, but drink too much, I start looking like an egg on legs. Wine waist is not a good look for me. Even more so when it comes with flaky skin and frizzy hair. Humpty Dumpty as a mad scientist, that's me.
So anyway, my point is, the things you put into your body don't just affect you in a linear, calories-in-calories-out, way. They have particular unique effects, that are different not only between types of food (or other ingestible substance), but also different between people. Maybe the sugar and the alcohol effects might be similar for most, but I bet processed-carb-turkey-neck isn't a mainstream affliction.
Good nutrition changes your body in more ways than one.
Watch what you eat, yo.