WRT the carrot cake, I will say that while there is only a minimal physical difference, there is a practical difference. Making a carrot cake at home is a commitment, and most people won't frequently go to the trouble except on special occasions... But one from the grocery store can be acquired casually and without effort, and it's easy to eat a lot more of something when it requires no effort.
Every time I cook (and thats quite often) I put a bit different ingredients that some factory would put in since they are the cheapest variant.
Some stuff is BIO, cream or coconut milk are lower fat version, or carrots are are without residual pesticides. Less salt since we use less salt, and taste buds quickly adjust so its still adequately salty, a better mix of herbs and spices so taste is.. simply better, more refined. We use with much less sugar, the same as for salt above (if you eat sweet stuff sparingly then even mildly sweet stuff tastes amazingly, just don't go from one extreme to another).
