If your water profile is good, you don't gain anything by messing with water chemistry. If your water profile sucks (which mine does), you have quite a bit to gain. My water is extremely hard, and full of chlorine, so I start with reverse osmosis water. RO water is too soft for most styles other than pilsners, so I need to add minerals back.
Check out the brew science forum here on HBT, there's lots of good info.