Yes, in the case of the Ops. Inc. muzzle brake, it's the "cool factor" since very few can acquire the suppressor that fits on it. However, the real deal Ops. Inc. 2-port muzzle brake has supposedly been found to enhance accuracy. I have one on mine, but I personally see no need for muzzle brakes on AR's other than the cool factor. However, my SPR is the real deal military issue so it came with one. They aren't even approved for post ban use.
Mine has the Douglass barrel, though Kriegers are used too. The Kriegers are a factor better than the Douglass. However, if you aren't going all the way with your SPR (fanatical about accuracy), don't feel you have to get the Douglass unless you can afford it. I have seen too many AR's shoot under 1/2"-3/4" without special expensive barrels (got lucky mostly). And yeah, I'll say it, I can't shoot as good as my barrel cause I know I don't have the skill - yet. It's so bad, I am having someone else do it for me right now. HAHAHA! Seriously... I want to see what my upper can do and I am not the man to do it.