Or to trigger moonbat liberals.
Meh - if a gun is still 'loud', it's a non-issue.
For some weapons like the MAC, the 'fake suppressor' gives the shooter something else to hold-onto to keep the weapon on-target, and provide a mounting surface for sights/optics. The original Ingram MAC-10 came factory-stock with a 2-stage suppressor. Without it, the gun was a psychotic bullet-hose that couldn't hit the broad side of a barn at full-chat. All MAC barrels are threaded for their easy installation.
I like them because they provide good esthetic 'camouflage' for extra-long non-sbr carbine barrels.