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 have heard that you can get the fattiest type, drain the grease as normal, and then rinse the meat with water in a colander to make it equivalent to the extra lean fat content. I'm not sure I buy that, and it seems this would rinse off any seasonings used also.
That sounds terrible. Cooked ground beef should be drained if necessary, but not rinsed. It will rinse away a lot of flavor, and I doubt it makes it the equivalent of having bought lean meat in the first place. There is still plenty of fat present in the meat itself.
If you want low fat get extra lean. If you want flavor get 80/20 or 70/30 and drain thoroughly for as little fat as possible. Do yourself a favor though and don't rinse it with water.
Best Answer
In general, when a recipe says 'discard,' it means that the part to be discarded is not to be used in the scope of the recipe. I see no reason why you couldn't save the beef fat for other recipes, it can be refrigerated for about a week or frozen for 2-3 months.
See this answer for tips on using the reserved fat. The sinew I would probably just toss. If you have enough saved, you could put it with animal bones to make stock, as the collagen in the sinew will break down into gelatin, but for me, it's not enough benefit to justify saving all those little bits and storing them.