Just the first example I pulled up to quote:
From KAK:
"KAK INDUSTRY "FLASH CAN". THE BORE WILL ALLOW A 9MM TO PASS THROUGH. THIS IS A SOLID 6061 ALUMINUM DEVICE THAT IS DESIGNED TO DIRECT MUZZLE GASES AND THE MUZZLE FLASH FORWARD. THREADED 1/2-28 FOR 5.56 OR SIMILAR WEAPONS."

I see a lot of them on 9mm SBR types of weapons.

Doesn't a properly crowned muzzle do that anyway?
Is a flash can supposed to do it so much better that I'd notice an improvement, making a follow-up shot faster/more-better/easier?

If it's a benefit on SBR 9mms, why don't I notice how bad my 9mm non-AR pistols are to shoot?

Thanks for any help.

I think they're pretty much useless. I would rather have more barrel length or a suppressor.

I could see it for a 9mm that you are waiting on a stamp for.  Makes the barrel long enough to be 16", added benefit of redirecting blast downrange somewhat, etc.

And IIRC, 9mm will be losing velocity out of a 16" barrel.....ballistics by the inch says that the fastest velocity of 9mm is right around 14" of barrel length, for instance.

Back in the early days of my time on ARF, short 5.56 barrels were common with perm attached long ass comps that were designed to look like suppressors (but weren't), or those moderator lookalike things......to make a legal length barrel.

Mine acts as a thread protector on the 9mm when the suppressor isn't On. 

Thread protector or muzzle device to use when not shooting with your can attached.

This. My muzzle sits an inch or so behind the end of my rail. With the can, there's no problem. Without the can attached, the KAK slimline flashcan brings the muzzle blast in front of the rail.

Thanks for the help, guys.
I feel a lot smarter now.

Same here.

Well, I guess I'm the oddball.  At first I thought the same; how is a blast can any different than a bare muzzle?  It made no sense to me.  Then I tried a KX3, KX5, and the MI Blast cans on some SBRs, a couple of .458s, 10 and 12 inch, and on a 8.5 inch .338 Spectre.  

I shoot under a covered range where the roof slopes down out to about 10 feet in front of you and about 5 and a half feet off the ground in front of you.  At 5' 10" I have to duck to walk out to set up the chronograph.

I don't know why, I won't try to explain it, but the concussion from those rifles when wearing the blast can is considerably less than when the muzzles are bare with nothing screwed on.  That sloping roof in front of you is murder when a brake or FS is used and is not pleasant when shooting a bare muzzle, but with either the Noveski or MI blast cans attached the concussion back from the roof is tremendously reduced, so they do work better at directing blast away from the shooter than even a bare muzzle.

I think it's primarily about the fat muzzle device sticking out from under the handguard look, which, although I think looks stupid and vaguely phallic, seems popular.

I was taking an indoor carbine class with a guy that had one and it was the loudest, most obnoxious piece of shit.  I don't know what brand of ammo he was using, but it was brutal.  We had the class split into two groups for one of the drills, and the instructor for my group sent the guy over to the other instructor because he got sick of it.

I have a 10.5" pistol build that has a KAK flash can and 12" m-lok rail. It is stupid loud, to the point of obnoxious from anywhere other than behind the weapon. And this is at an outdoor range. I personally like the look, but to be honest at some point I am going to put a 9" rail on it and ready it for a silencer. Especially since it is going to be my home defense/shtf gun.  No way anyone in the house would have ear drums left if I fired that thing down a hallway or anywhere in the house.

IMHO they're just to take the place of a suppressor when not in use. Protect your threads and if your barrel is shorter than your rail it keeps 'em clean.

For blast/concussion management a linear comp is usually more effective.

My 9mm pistol upper came with a 4" barrel in a 6" rail with a KAK can. Unsuppressed I'd be better off with a 6" barrel.