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.
I very quickly found nutritional information for storebought (Swanson) chicken broth and chicken stock. As you'd expect, neither one contains all that much protein. The stock does contain substantially more: 4g per cup compared to 1g - this should also be expected, since stock has some gelatin in it, and gelatin is mostly protein.
Of course, it's commercially made; I imagine you'd be hard-pressed to find good nutritional data for homemade stock and broth.
Best Answer
Yes, it is really chicken fat rendered during the stock making process.
Called schmaltz in Yiddish, it is an ingredient in its own right. For example, you can use it to fry foods, or instead of butter in creating a roux, when you would like the chickeny flavor it provides. It is a key ingredient in matzo balls, and similarly, makes spectacularly good dumplings of various sorts. You can refrigerate it for several months, tightly covered.
It is not necessary to return it to the stock, although you may use it as an additional ingredient in the dishes you make with the stock.
Stocks are normally defatted anyway, so you do not need to return it to your stock.