Typically, that is how “most” are advised to fuel because during aerobic exercise , your first source of energy is to break down gylcogen to get glucose. So “carb loading” essentially maximizes your glucose storage ( glycogen) in muscles so that you have enough to power your workout. After your workout, your muscles have depleted their glucose stores, however, muscles also tear as a result of exercise. To help repair the muscles into even stronger muscles, a mix of carbohydrates and protein is suggested because carbs help your muscles utilize the protein for forming the muscle repairs.

I am the opposite… I almost always want pasta and carbs after a long run and I almost always want to have protein before. I don’t really understand it for myself yet, but I have noticed this difference particularly since last september/october when I had began running again.