Personally, I see how it's almost superfluous, but it does fit in well with later game full powersuit scenarios. I guess it's a YMMV thing.
I guess an alternate example/reason would be the Doom chaingun, and other similar weapons. It's a sorta not-AR or not-SMG for people that want to be specifically non-standard like I guess 90's style or something.
In my particular case I kinda wanted to have a visual distinction between calibers of miniguns.
Shorter would kinda imply it's dismounted from a smaller vehicle than the larger sprites would. I have seen a helicopter mounted minigun in person and they are scary-long. I believe many are .308 but smaller vehicles get 5.56 sometimes, like humvees? I may need to find better webpages for resources if not...