It comes from the days when people who lived away from coastal areas sometimes had problems with goiters because they had no iodide in their diet. It's inexpensive, commonly available, and the one most of us know. Kosher salt also comes from the land but its structure is different - a grain looks like a snowflake whereas a grain of regular iodized salt is shaped like a cube. Kosher salt is very light and is less salty tasting because there's less of it per measure. Connoisseurs of salt like to cook with it because it's not expensive and has a pure salt taste.
Sea salt is from evaporated sea water and the flavor varies according to where it comes from.
As the sea water begins to evaporate, the first crystals that form have a very fragile structure and a delicate, sweet, complex flavor. Taste and try sea salts because you'll find a lot of good ones available.