I don't use Mark Points often, but they sure can be fun. If you find a bunch of static tanks, for instance, try making Mark Points on a few of them and learn to cycle through the Marks. There's a training mission - Sitting Ducks, that you can experiment with.
This works, too - finding an air defense threat that is displaying on the RWR. Set the TAD in Depressed Mode
to get more real estate on the TAD screen. Line the jet up with the RWR symbol and set the TAD to 80 miles. Make a Mark Point right in front of the jet and then another mark Point out about 70 miles.
Take the jet off to the left or right > 30° and make a second set of Mark Points like the first. Then you'll need to turn these four points (or more, depending upon how many times you inadvertently press TMS Right Short) into a flight path, I forget how to off the top of my pointy head, and where the AB line crosses the CD line is where you should be able to find the air defense threat. That is cool stuff.
You can also make a Mark Point, or two, at your present position so you can find it again, which is what - Ice is saying.