It's not a gimmick. As has been stated, it shows the capability of the rifle - not the shooter.
I have a rifle with a >0.5 MOA guarantee. It came with two, 10 shot test targets, the large spread was 0.375 inches.
I have been able to duplicate the accuracy under the following conditions:
1. 100 yard tunnel (no wind)
2. Sandbag rest
3. Sandbags on top of the barrel
Now to how practical it is - can the shooter actually use the accuracy? What the performance does is rule out the equipment as being a problem. Anything you miss is purely your fault as a shooter, and not associated with the rifle.
From a mental standpoint - that takes one variable out of the shooting process.
I use my rifle out to 1,000 yards shooting steel targets, and have used it for precision tactical rifle competitions - and for one stage, they put clay pigeons at 400 yards as the targets. Since the event is timed and your score is a combination of accuracy + time, any help you can get in the form of a more accurate rifle is a bonus.
Lastly - an extremely accurate rifle can make you a better shooter. As an example, I took my neighbor shooting. He's not a shooter and has very little experience with rifles.
I was the spotter for him and setup the scope for elevation, and then called the windage adjustment directing him on how many lines to hold outside of the target using the horizontal MIL reticle markings.
He hit 4 out of 6 shots on an 18-inch steel target at 1,000 yards - that's not too bad for an inexperienced shooter. It was the inherent accuracy of the rifle that allowed that type of performance with a little coaching and help from me.
If you've never shot a precision rifle under controlled (range) conditions, you would never believe the difference the equipment can make in helping you shoot better.