Hey!
Barrel length does affect both accuracy and range, but only to a certain extent.
If you pit an 8" up against a 14", then the 14" is gonna outshoot the 8" with better accuracy at longer range.
But the difference is minimal and isn't much of an issue if you're a fontman, or even a middleman.
The barrels that go beyond 14" aren't all just marketing gimmicks, although those 21" out there are.
Longer barrels such as 16" and 18" are aimed mainly toward use for airballers who use the extra length to push into a bunker to minimize the amount of themselves that they have to expose to the opposing players.
But often, newer players buy the longer barrels because they're under the impression that the longer the barrel, the better.
Another thing to keep in mind is that, a 12" or 14" is going to be able to hold more porting than an 8" or 12" thus making the shot quieter.
Don't be played, do research on any barrels you are considering buying because paintball's expensive enough without having to waste money.