The ported barrel reduces muzzle flip a little bit by directing some of the exhaust gas up, preventing the muzzle from rising. Useful in competitive scenarios where getting the sights on the next target quickly is important.
One downside of the ported barrel is the exhaust gases tend to dirty up the front sight. Not a big deal with black or fiber-optic sights, but the standard white dot gets duller and duller over time unless you clean it often.
BTW - I have a ported Storm Lake 40-9 conversion barrel for a M&P FS I'm looking to sell. Due to the sight issue mentioned above, I bought a non-ported barrel and don't use the ported one anymore.
