As stated above, it's to get the shortest legal configuration without having file for an SBR stamp. I've settled on the following for my primary 5.56 ARs:
18" SPR for hunting & target shooting
14.7" Recce p&w for my "do-it-all" gun
11.5" Pistol for CQB/HD
I've found 16" and 18" to closely overlap in terms of performance & handling, so I prefer the extra velocity of the 18" for hunting purposes. The 11.5" is ideal for short-range use and still feels light & handy with a mounted light. The 14.7" splits the difference between the two with a combination of compactness & reach (with the right ammo) that is very hard to beat. It may only be a few inches shorter than a 16" with muzzle device, but it's my ideal compromise.