I'm asking the same question myself.
A video I watched (may be the one you linked) showed SMG spreads to more straight upwards rather than up and to the right. It also kept the spray closer together (to the center).
I read somehwere that foregrip brings the gun back to center faster.
If my assumption is correct, this explains why tests show a slighter tighter cluster on full burst wall models, and why they're clustered more spherically. For an SMG this means after every burst it will pull back to original position faster, meaning it 'strays' less and even when it does stray it holds the bursts in a more linear line (tighter back to the line even if it is straying).
I can only assume then that it is more useful for guns that are shot in bursts, or guns that have shorter sprays (Assault Rifles). It will increase accuracy because it brings the gun back to its original position faster, reducing 'straying'.
I'm not sure if this is correct though, it's combining information I've read together to explain what I've seen in videos. I think that it's slightly useful for some SMG's (the ones that when tested, showed circular patterns) but I have now stopped using it until I see more information.
I think that for specific weapons, say the Type 25, where range is longer and timing between bullets is longer (more time to pull back to center) it allows for an more noticeable increase in accuracy (bullets fired closer to the original position of first placement). I think this would also improve the FAL with auto-fire, as timing between bullets is longer.
Does anyone else have any input? Link here if TMartn posts a vid with more detail on its usefulness.