There was a special on the Military Channel on these types of weapons. Granted there's a difference for civillian semi's - but their purpose was an entry weapon that could penetrate armor with the 5.56 round, instead of the classic Mp5's they've been using.
For us sivies - NO CLUE what the purpose/market is there. (Home defense against bad guys with body armor?...)
At least with the .308 hand-cannons from Savage you can go hunting... Not sure what you'd hunt with a 5.56 though lol