So I've got a friend who makes many dishes involving hamburger but doesn't ever strain it. The grease is added to a dish like Shepard's pie which to me just seems very gross. I see alot of posts about how to strain but nothing to discuss why you should/shouldn't strain and what you should do as an alternative if the grease must be used.
Well, all I can say is 'it depends on the dish'. If the dish calls for fat to be added otherwise, you can keep that fat and count it where you would add some later. If the dish is just adding hamburger and no more fat - I agree, discard it.
That said - I don't find hamburger fat particularly flavorful and nearly always strain it and add another kind of fat I like better.
It also depends on the fat percentage. If I'm using lean 97/3 hamburger, there's hardly any fat to drain.
There are legitimate reasons, and one is because you loose more than just the grease if you strain it, as the meat will give off other liquid.
Instead, the way I learned to do it is to push the ground meat to one side or make a sort of well where the liquid will collect (and you can tip the pan slightly to that side, if you need to), and then collect the grease on top with a spoon or ladle, leaving the juices at the bottom.
Of course, this assumes that you want the moisture in whatever it is that you're cooking.
And, my other reason is that I hate washing my strainer. (people who have dishwashers might not care). If I really need to drain it all, I put a loose fitting lid on, and hold the lid on with one hand while draining the liquid into my grease can ... but it doesn't get 100% of the liquid out.
And that made me think of one other aspect of draining grease ... NEVER do it down the sink. Always do it into some container so that you can keep the fat from going down the drain. Restaurants in most places in the U.S. (and maybe in other countries) have grease traps to keep fats from entering the sewer system ... not doing so will result in the fats solidifying inside your pipes, or in the main sewer system. Boiling hot water may loosen it up in your area, but will just flush it down further, until it becomes a community-wide problem.
(About 4 years ago, there was an incident the next street over, where 5?6? homes had their plumbing back up and basements flood thanks to grease in the sewer line).
You want to put grease, especially that from animal fat (as it solidifies closer to room temp) into a container that you can either (1) let solidify and toss out with the trash) or (2) seal up and toss out with the trash.