looking at it logically, there are two reasons to shorten the barrel on a rifle or simmilar "shoulder arm" to that kind of length:
2) manuverablitiy in close confines (vehicles, buildings ect)
concealment seems very unlikely. unless you are planning to hide it in some kind of bag, you just can't hide a beltfed under a jacket.
manuverablity in close confines seems a little more likely. but why?
it doesn't make sense for FIBUA, the belt would just get in the way.
my best guess for how it could make sense in the real world is for vehicles. either for providing heavy firepower from vehicles not orgionally intended to fit heavy weapons, maybe smaller hellicopters, or where heavy weapons cannot be outwardly shown (alledgedly the protection team for the US President have a minigun type weapon hidden in one of their vehicles, maybe they would have a use for it) It could also have value for use with the firing ports on an APC or simmilar armoured vehicle, where traditional full sized MGs are just going to get in the way.
but honestly, I suspect it is an american thing, where those that can afford the incredably high price of a belt-fed fully auto firearm figure they might as well have the short barreled version, since short barrels have simmiar legal barriers to ownership as the full auto firearms.
edit: just noticed it's semi-auto only. only way I can make sense of that is that it's to get around some kind of magazine capacity law......
the comment in the thread linked to about it missing "the happy switch" and then talking about NFA legal sears.
I'm assuming they've used an origional full auto trigger unit, that's been fitted with a semi-auto sear, to comply with US regulations. god only knows why you would do that, unless you can't get semi auto only trigger groups easily.
IIRC, some chap called Fleming (FR Ordnance?) decided a short barrel, fully automatic variant of the G3 would be a spiffing idea. After several SF chaps attempted to use the thing in Northern Ireland and came back with impared hearing and their splendid 'taches somewhat singed it was concluded that the idea was utter horlicks.