The longer the gas system, the softer the cycling of the action. This can reduce recoil and smooth out the firing. More gas is expanding inside the barrel vs exhausting through the gas port. The .308 AR is notoriously OVER gassed and the carbine gas systems (on the 16"+ barrels) tend to have some cycling issues, creating failure to feed problems. In some cases the action cycles so fast, the carrier ejects the casing so fast that it returns to battery faster than the next cartridge can be lifted in the magazine.
The .308 cartridge has a LOT of powder in it. I honestly believe a rifle length gas system would work on a 14.5" barrel... maybe even a 13.7"... with the right gas block.
Piston operated systems, when timed and tuned properly, don't have the same issues. In fact, when made properly, the piston can be significantly shorter (even pistol length) as the gas system regulates the amount of pressure it wants to use. A baffle system prevent excess gas exhaust. An extra long piston set-up can get be significantly heavier than a direct impingement system.