Stir fries, stews, sauces, soups. Also good for stuff like deep frying batter/breading, sprinkled on vegetables, sprinkled on hamburgers along with salt & pepper when you cook 'em up, etc. Any type of “all purpose seasoning” or “seasoning salt” application will do well with a bit of MSG.
That makes it sound like I use it in everything, but I actually don’t. I do use it about once or twice a week in what I cook. Once you get used to it, you can identify the MSG taste, or rather that kind of full, savory feeling it leaves on the middle back of your tongue (or at least that’s what it does to me.) Just taste it straight again, like you did, and really pay attention to the flavor, your tongue, and how the flavor lingers. There’s a bit of saltiness to it, but it’s not the sharp stinging flavor of sodium chloride. It’s that umami flavor, which is deep, savory, meaty, but, when overdone, can be a bit, I dunno, overwhelming tasting. You know, kind of like salt. Overdo it, and everything just tastes salty, but at lower levels, it brings the flavors together while adding a bit of that saltiness we crave. Also gives food a “processed” taste (just because it is used so much in processed food) at higher levels.
I use MSG at lower levels than salt (and you have to balance these two, as both contain sodium, so MSG does add some saltiness to the dish, as well as the umami). Basically, start with a low amount (maybe 1/4-1/2 tsp for a dish for 6-8 people, taste, and build up from there if you need to.) And if you’re using stuff like canned chicken broth or chicken bouillon cubes, you’ve probably already got a good amount of MSG in your dish (though it can go by several different names.) And, yes, you have your glutamates in tomatoes, mushrooms, cheese, seaweed, etc.
Unlike salt, though, the one thing I’ve never tried using it in is in sweets. I have no idea how it would react with that. Salt is pretty typical to add a small amount to sweet dishes. I’ve never tried the same with MSG. In my mind, it doesn’t work, but it wouldn’t surprise me if it does in low doses or some specific applications.