I only really simmer two things that I cook for so long.
Spaghetti and chili.
I simmer them for so long to allow the herbs and spices to put off their full flavors. I picked fresh oregano from outside, and chopped it up and added that, and since it is so fresh, it needs time to let all the flavors gel together. Also, the veggies need plenty of time to get tender.
Chili on the other hand, I use a London broil, or a chuck roast to make chili with. It simmers all day to allow the meat to become very tender.