It's a mount. For a handgun. So you can affix it to your long gun.
One more time: if you have a rifle, or a shotgun, that has a standard Picatinny rail for mounting stuff, you can choose to mount a flashlight, or a laser sight, or a forward handgrip, or, now, thanks to the folks at Mako Tactical, A PISTOL.
Lemme 'splain: in gun-nut circles, we make fun of guns that have too many accessories mounted on 'em. This is a long and glorious tradition. We know there are Tactical Tommies driven to put everything on their AR-15s but bumper flaps, and we joke about it. But this one... it really is the topper. If I'd seen this a couple weeks back, I'd've called it an April Fool without hesitating. I suppose they get points for turning it up to eleven, but the UTA seems like a solution in search of a problem. If you have a long gun deployed, you pretty much do not need a pistol, because the long gun will kill anything the pistol will, and will kill it deader. I can maybe see their point about switching from suppressed pistol to loud-n-scary rifle, but that's about the only practical use I can think of, and it'd take a hell of a lot of training to not screw up.
I can see it being something to own just for the sheer stupidity of it, for playing around and slaying fierce pumpkins or something. Especially if you have access to stuff restricted for us common folk. It could be outrageously moronic fun, say, if you've got a full-auto rifle with a full-auto pistol mounted under it and a vicious pumpkin patch that's threatening our precious bodily fluids -- firing TWO FULL-AUTOS AT ONCE, c'mon, how stupid and awesome is that? But that's out for the regular joe. And besides, they're not selling to regular joes; sales are restricted to military and law enforcement. You know, people whose jobs involve life-or-death situations where they might actually have to *use the damn thing.*
Although, looking at it again, I just realized the stupidest thing about it: judging from the picture, it looks like it'd get in the way when you go to change the rifle magazine, or at least make it harder to get a good grip. Well, that'd be one reason you you need the mounted pistol -- if you shoot your rifle dry, you're screwed.