It's a fluid retention thing: http://www.sparkpeople.com/resource/nutrition_articles.asp?id=166
This can be harmful to people with high blood pressure or heart, liver or kidney diseases. People with these conditions should be especially careful about sodium intake. But there’s some debate on whether everyone needs to worry about all of this salt talk. We’ll listen to the USDA, who recommends that we need to choose and prepare foods with less sodium. The average American adult consumes about 2,500 to 5,000 milligrams of sodium a day. But we only need 1,100 to 3,300 milligrams, or about 1/2 to 1-1/2 teaspoons. That can be a pretty big difference.
One of the main sources for sodium is the processing of foods. It's in not just canned foods, but also in frozen ones as well. Lots of manufactured diet foods are salt mines as sugar is removed (cottage cheese is very salty). Restaurants are also heavy with the shaker.
Cutting salt intake is good for dieting, not just so that you can release fluids (heh, do #1) as all that liquid weighs something and makes you bloated, but it also means that, by definition, you're generally eating fewer processed foods and more fresh ones, and you're most likely going out to dinner less, where portions can be outta control and ingredients are unknown at best and downright awful at worst.