"What's the point of shooting a round at a reduced velocity knowing that isn't going to preform like it should? "
Silence.
I don't think it is any revelation that a jacketed bullet at 1100 fps isn't going to perform like it would at 3000 fps. That should be obvious. But appearently what isn't obvious is that the bullet still puts a hole in the target.
Let's say you are walking around with your AR15/M16 rifle loaded with your favorite varmint load. Your primary goal is to call coyotes but at some point you see a few squirrels and decide you want to bag them for dinner. You don't happen to be carrying around a couple spare .22LR or 9mm uppers so instead you reach in your pocket, pull out a magazine of subsonic .223 ammo, do a mag change and you are good to go.
Let's say you are on a tactical team getting ready to do a raid on a house. You notice that the occupants have a dog outside that you know is going to give away your presence to the people inside and ruin the element of surprise. You look around for your 9mm upper or your .22LR upper but realize these things only exist on internet gun forums. So, you do a quick mag swap replacing your regular duty ammo with subsonic .223. You knock off the dog and no one in the house is any the wiser. You do another quick mag swap and you are ready to kick in the door.
Let's say you are a bad **** operator.
You are doing some dirty deeds in enemy territory. You lay up during the day and work at night. Everything you need to live and survive is contained in your rucksack on your back. You meant to bring two or three other uppers with you like you read about on the internet but as an experienced soldier you realize that carrying 700 pounds of gear is only for the guys on-line. As you attempt to remain undetected, someone comes along and you realize they are going to eventually see you and give away your position. Again with the mag. change.
Let's say you are a mild mannered US civilian that owns and AR15 and a suppressor. You live outside of town but you have neighbors 50 yards or so away. You have some raccoons that raid your garbage cans every time you set them out for the garbage truck. You would like to be able to choose from a dozen or so different weapons but the only one you have that is suppressed is that AR15. You consider spending $700 or so for a different upper but decide instead to simply handload a few subsonic .223 rounds.
Let's say that you are a typical gun enthusiast that enjoys guns, shooting, and handloading. You own an AR15 and buy a suppressor for it. Because you enjoy handloading you think it would be fun to work up a good subsonic load for your AR.
Let's say you are a guy that owns 9 different suppressors. You have four different .22 suppressors as well as a dozen different guns threaded to accept those .22 suppressors including .22LR conversions for your Glock 17, your Glock 26, your 1911, your Browning Hi-Power, your Colt Officer's Model, and your AR15. So if you want to shoot a suppressed .22, you got that covered. You also have an LRM M169 suppressed 9mm upper, so if you wanted to shoot suppressed 9mm from your AR15s you got that covered also. But you don't want to do that. You want to shoot supressed .223 with subsonic ammo.
So you do.
That isn't so hard to understand is it ?
Some people just want to shoot subsonic .223.