Um, the main purpose is to add taste to the food. It's usually a subtle difference, if it's noticeable at all, but sometimes it can add quite a bit of flavor, like with plain noodles. Most of the time a person wouldn't notice the difference because the noodles get covered with various sauces that have their own taste, but try eating just plain noodles with and without salt added to the water and you'll notice a difference.