Salt is a flavour enhancer - a little wont hurt most beers. Fourstar's info is right, under 150 ppm they enhance and round out malt flavour.
A teaspoon of salt can add a little palate fullness and grunt to the malt backbone of anything from a light lager to a bitter. Just be careful that you aren't loading up the chloride/sulphate balance too much so it doesn't end up sweet. A handy trick if you are looking for a really malty Helles, or maybe a vienna or ocktoberfest.