The veggies aren't very palatable after such a long simmering (unless you like celery paste, I guess), but if you've thrown a whole chicken in the resulting meat is wonderfully tender and flavorful.
Rendered beef fat can be used in a lot of ways. You can use it in place of oil in a lot of recipes, but finding out which ones you like will take some experimenting. Around our house, I use rendered fat from beef or bacon in place of oil when sautéing, for example with onions and peppers, garlic or mushrooms. I've also used it to add some kick to gravies.
You could use it to pop popcorn, which I've heard is delicious but unfortunately have never tried.
I've had pasties made with beef fat, and they were delicious. Mashed potatoes as well.
As far as fat ratio goes, it's better than butter, but not as good as other fats. Beef fat has a high smoke point and is suitable for frying.
The table below is based off of 1 tablespoon. Ratio means saturated to unsaturated. Smoke point can vary depending on a lot of factors (olive oil can range from 300 when unrefined, to 375-450 when refined depending on quality) but the table below should be a good guide. For the oils, I took the refined numbers.
sat mono poly ratio smoke
Canola Oil 0.9 8.2 4.1 1:12 470°F
Olive Oil 1.8 10.0 1.2 2:11 450°F
Chicken Fat 3.8 5.7 2.6 1:2 375°F
Duck Fat 4.3 6.3 1.7 1:2 375°F
Lard (pork fat) 5.0 5.8 1.4 5:7 365°F
Beef Tallow 6.4 5.4 0.5 1:1 400°F
Butter 7.2 3.3 0.5 7:4 350°F
(fat source) (smoke point source)
A lot of people mix it into their dog's food, or use it to feed birds.
Seach for tallow if you want to find recipes that specifically use it.
Best Answer
It's odd that they force you to choose one, as many things in cooking have multiple reasons, e.g. browning meat adds flavour and colour to a stew. Are you sure the question isn't one of those 'tick all that apply' ones?
In this case, the tomato paste adds flavour, colour, and the acid helps break down the connective tissue in the bones, which helps the stock to jellify.