I saw this in my local gun shop a couple days ago for the first time, stared at it for 10 minutes trying to figure out just what the heck it is and what it does. I see absolutely no useful purpose for this.
what it does:
1) allows multiple accessories to be mounted
2) allows it to be cocked from the left side
3) gives you something to hold onto at the front for stabilization of the gun
4) adds a considerable amount of weight
5) forces it to be carried via lanyard, as there is absolutely no other way to carry it
6) converts your 9mm sidearm in a very clunky and heavy 9mm (or .40 to .40, or .45 to .45)
7) improves accuracy by 0%
it basically eliminates all of the "pros" of a handgun, and adds all of the "cons" of a rifle, yet still only has the same exact power as a handgun.
you might as well buy an AR, use subsonic ammo, remove the dust cover and forward assist, and duct tape a brick or 3 to the receiver.
this is a gimmick, don't be fooled. I'm actually extremely surprised Sig of all companies would be the one to come out with this