Because you want to be able to log the calories/nutrients of the food you're eating before it's cooked into a dish.
You don't want to have to cook your bacon and then measure its mass before you know how many calories it has when you can just log the raw form before you cook it.
You don't want to do that, but unless you are consuming all the rendered fat that's what you do to get an accurate value.
Sure you do. And if you don't consume all of the grease, then you have some overestimation error in your tracking which is evened out by (but usually dominated by) all of your underestimation errors, like every time you forget to log the squirts of oil in the pan or that handful of mixed nuts or every time you eat out.
Having to cook your food first, take it out, measure it, and put it back in the dish you're making before you can estimate content doesn't seem like a recipe (pun) for habit forming here. Nor is it viable for anything but the most basic dishes like individually pan frying large ingredients.