The whole point of sweet corn - and it's essentially what separates us from the livestock that consumes much of the "dent" or field corn grown in this country (that and the whole cud thing) - is its sweetness. And if you've eaten much corn, you know that that sweetness can be a little unpredictable.
In the days before supersweet and sugar-enhanced corn varieties became widely available, adding a few tablespoons of sugar to the cooking water provided a little extra sweetness to the corn. And since no one these days knows whether the corn they've purchased is an enhanced variety or a traditional variety of sweet corn, many people still add sugar to the pot.
Some people find a combination of sugar and a supersweet variety to taste artificially sweet, so you may want to test-cook one ear before committing to a whole pot of doubly sweet corn.
Shirley Corriher, a wonderful Southern cook with a great appreciation for flavor (and author of Cookwise), sweetens her boiling water with a third of a cup of honey.