Let me present a CONCEPT here:
http://www3.telus.net/gamegetter/operate1.html
The story with that goes: this guy is a Canadian. He hunts sheep in the mountains while living out of a backpack for extended periods. Everything he has, he carries on his back. He wanted to supplement his diet with fresh meat in the form of small game. However, his big game rifle was unsuitable for shooting small game. He couldn't carry a .22 rifle, or a shotgun. And appearently in Canada, he also couldn't carry a handgun. So he developed this cartridge adapter to enable himself to use his big game rifle, to shoot small game animals.
So, lets apply that to what we are talking about here: let's say you are carrying a rifle chambered for .223/5.56 for some reason. Whatever reason. And you also have a need for something smaller. Yeah, you could carry around a golf bag full of rifles so that you could perfectly match the cartridge/rifle to whatever shot you want/need to make. But, that isn't realistic. Instead, you tailor your LOAD to suit the shot/target you are firing at. This is what handloading is all about FWIW. Changing the whole complexion of the rifle you are shooting is as simple as changing magazines. By using different loads, you can simulate having various different guns, only you don't have to carry them.
There are MANY other possible senarios where you might not want to shoot a full powered round: noise, recoil, maximize the effectiveness of a suppressor.......................... Use your imagaination.
FWIW: when I was loading subsonic .223, I was using a 77 grain, Sierra Matchking bullet (as I mentioned previously). I was shooting it at something like 1050 fps. A .22LR shoots a bullet roughly HALF THAT WEIGHT at approx. the same velocity (depending on barrel length, rifle vs. pistol, brand.....). So for those of you comparing subsonic .223 to .22LR arn't grasping the facts.