Why do you stir italian sauce all day
To cook a sauce for a long time, particularly without a lid, concentrates the flavor of the sauce as the water evaporates. That's also called reduction. Yes, sauces that are cooked that way need to be stirred frequently to avoid allowing them to stick to the bottom of the pan. Sticking is bad enough, but it leads to burning, which is worse. 

In meat sauces (sauces that include meat), long cooking can also contribute to tenderness of the meat and allow the meat flavor to permeate the sauce as the long cooking breaks down the collagen in the meat.