The Soviets tried out issuing SMGs to tank crew and rear line personnel. But they found the lack of range and armor piercing capability of 9mm rounds made them too much of a liability. So they issued troops the AKS-74U instead.
And then they set to withdraw them from service as the Krink is pretty awful a weapon. The Short barrel and abbreviated gas system creates a number of issues of overheating, poor accuracy and generally poor performance. In its place the Russians would eventually develop the AK 105 and AK104 with a 12.5 inch barrel.
This is fairly similar to the AR15’s history where in Vietnam numbers of the Colt model 607 and XM177 CAR15 commando would be issued but found obnoxious as a weapon due to High sound flash and poorer reliability vs the standard M16 of the era. Though the shorter length made it more handy the muzzle blast and sound report made it a double edged sword vs both its targets and any armored vehicle crew who we’re supposed to use it. To correct this various modifications were made including increasing the barrel length to 11.5 inches (though the current Mk18 returned to 10.5” barrels) or adding a moderator increasing it to 14.5 inches (M4’s barrel length). Very few assault rifles in military use prove reliable or are fielded with barrel lengths below 10.5 inches. The AKS74U was about a 9” barrel as is the G36C which isn’t widely used beyond airsoft and movies props.
I think a good question is why bother with this SMG in military use when a carbine like QBZ-192 would have a similar form factor and way better capabilities including range and penetration. Not to mention it wouldn't make logistics more difficult by adding yet another ammo caliber to the ground forces.
You end up answering your own question. However in comment on the latter the PLA seems to be dropping the 5.8 pistol caliber entirely for 9x19mm which is common to the SMGs meaning that that they are a common caliber.
My guess by looking at the advanced age of some of those people and the insignia on their collars, that these SMGs will be a replacement for pistols for officers. These guys are ranked Colonel, Lieutenant Colonel, Captain and the like. Even a couple Major Generals. Or at least that would have been my guess if it wasn't for the fact a lot of them also carry pistols.
Which is similar to what is seen in a number of armies. Where SMGs and PDWs are issued to rear line troops or troops in positions where a rifle is to big or where it’s just inappropriate.
When we saw the QCW171 in parades we saw it on the hands of tank and IFV crews where their primary weapon is the vehicle and the SMG would only be used if someone tried to mass it or they had to bail out.
The Russians have adopted SMGs for use by pilots as a survival weapon. A situation where in space of the ejection seat survival box is the limiting factor and let’s face it you can cut down an SMG far smaller than a rifle.
The US has retained SMGs for personal protective details around General officers in diplomatic missions rather than combat missions.