1) Ability to put a chain guard/case on it to keep pants clean and reduce the chance of them getting caught in the chain/crank.
2) Gears sealed off from the weather-less chance of wear from dirt and grit.
3) Pretty solid reputation. The wheels aren't dished, so a bit more strength.
4) You can shift without the need for pedaling. Great for those moments when you get caught at an unexpected light.