While the short barrel has a certain appeal, it really doesn't do anything significantly better or worse - except take more money out of your wallet getting the stamp or pinning the flash hider.
Although the 16" limit seems an onerous restriction, what it does best is generate revenue.
Very few carbine owners need the shorter barrel to negotiate hallways or interior corridors in CQB, vice versa, few shooters would see much difference at longer ranges with increased speed and more reliable bullet expansion with hunting rounds. It boils down to the "cool" factor, do you want to spend extra to say it's an SBR, or nearly?
There's such a small incremental difference in performance I didn't bother on my build, save the money for more ammo or a nicer red dot is what I choose. The finished gun will perform better and so will the shooter. It's like looking at it from Uncle Sam's view, what do you really get for the money?
All those M4's in the inventory and the Marines still issue the M16A4, the Army put 20" barreled rifles back into the hands of better shooters as the SDM. Some considered the overall results from M4's weak enough to justify inventing the 6.8SPC to compensate the reduced performance of 5.56 from a short barrel. That's been a popular choice for the 16" barrel as it gives 40% more power.
Just another way to look at it -
