Why is it important to add salt during cooking?

A few general reasons:

• As soegaard says, the salt is often better distributed through the food if it's added during cooking. This is especially true for foods which have been carefully assembled; you can't always just stir things up! Some foods also take a while for flavors to soak through; think about potatoes in a chunky soup. If you add salt only at the table, the liquid will benefit, but the potatoes will generally taste bland, like they were unsalted. Then there are things where it really needs to be cooked in - I doubt many people want to sprinkle salt on a chocolate chip cookie.

• Sometimes the quantities need to be pretty small, so mixing a small amount into the whole recipe is doable, while if you tried to add it at the table, even a little sprinkle would be too much. Baked goods are a good example here too.

• Virtually everyone likes at least a little salt, so it's usually quite safe to add at least a little. The food will end up tasting good to many people, and then some can add more. Often the groups of people that we eat with have similar enough tastes that it's a non-issue.

• Sometimes salt actually affects the cooking process, not just the flavor. It can help draw moisture out of sweating vegetables, softening them faster. It's abrasive (before dissolving, of course), and can help denature proteins.

But of course, if you're eating with people who don't like much salt at all, and salt can easily be added at the table (a pureed soup, for example), then there's nothing wrong with waiting until then.

The main reason salt is added during cooking and not after is to ensure a better distribution. This makes every bite taste as intended by the cook.

Besides the above entries;

Adding salt later brings the taste of the salt forward and masks the taste/flavors of the meal.

Here are 2 more reasons salt is often added during cooking.

1. Meats: It dries the surface of whatever you are cooking, like when cooking meats Brazilian-style. Salt in significant quantities is a desiccant and thus contributes to the "cooking" of meats and other moist proteins.
2. Baking: Incorporation of salt into baking doughs during preparation and/or cooking helps regulate the leavening process by counteracting alkalies (baking soda), and acids (baking powder). Salts are produced when acids and alkalies react.

Here are some reasons to salt after cooking.

1. Salt protects the flour and oil from going rancid due to bacteria and it may slow the growth of yeast used for leavening in bread dough. But this is a reason to put salt on the surface of doughs after preparation and before baking.
2. I find that salting after cooking (except for baking dough or cooking meats) allows the use of much less salt (for health reasons) while retaining the same pleasant salty flavor and preservative benefits. Maybe it's my imagination, but I imagine salt on the surface helps reduce bacterial contamination of food as it sits with its surface exposed to air at room temperature before serving.
3. Your tongue cannot taste the flavor of particles incorporated deeply into compounds and mixtures, so you'll only enjoy the taste of the few salt particles that reach the surface exposed to your tongue, so for maximum "bang for the buck" you want all seasonings on the surface.
4. Of course, when you salt the exterior, the salty flavor is "courser", less evenly distributed. I prefer this "flavor texture" to the homogenized saltiness of processed foods.

I do think the main reason is that it's better distributed but I'll add just a little something: Osmosis!

If your water is not salted then the minerals inside your ingredients will be released in order to have a balance with the water. On the contrary if the water is too salted, then your ingredients will absorb the salt, and then it will be too salty.

Usually I don't like that much salt, but when it's come to boiling, I fell like I need to add a lil' bit of it. Otherwise, the food seems to have lost some of its taste.

Not only does the salt allow for more even distribution of the salt, but salt has the ability to bring flavors out.

This can be illustrated by sautéing veggies or searing meat. If you give the ingredients a little kosher salt, you can really change your dish around. (Especially kosher salt since it has a lower sodium level that other salts, therefore more curing and less salty)

A great example of this steak. Before searing a steak, give it a light dusting of kosher salt and let it rest for ~15mins or so.

When you throw it in the pan, you'll notice a delicious crust forming. That's because the kosher salt brought out lots of protein laden water from the steak to the surface.

Hope this helps.

EDIT: A bit of clarification about "kosher salt being less salty".
It's not actually that Kosher salt has a different chemical compound, but if you measure it by volume, you can get some drastic differences.

For example:
Hain Iodized Sea Salt - 1/4 tsp - 1.5g - 590mg sodium
Diamond Kosher Salt - 1/4 tsp - .7g - 280mg sodium

That's a 200% difference.
Of course if you did it by weight, like a real chef/baker there would be no difference, only the size of the crystals.

This is why I said that Kosher salt is a less salty salt. ::tonguetied::

Chemically, salt is just salt, that is sodium chloride. Salts from different sources may have different degrees of contamination, but these are small and pretty well irrelevant in the context of the dilution in a dish of food. Salt is very readily detected by the tongue on impact and that is why the salt that comes in large flakes - and added after cooking - is prized. Add flaked salt after cooking - you'll get the impact without the health repercussions of excessive salt intake - the rest is marketing hype. (I exclude of course things like breadmaking where salt is integral to the process).

The short answer: seasoning. We can only taste four (or, five, depending on whether or not you consider umami) flavors: salty, sweet, bitter, and sour. Without any of these, food will taste bland, no matter how it was prepared. The best tasting food usually combines all of these in some ratio, depending on which flavors the cook wants to emphasize and which flavors will play a supporting role.

Salting during cooking is a subtle technique to season certain ingredients/flavors and intensify their flavor. Thomas Keller has a good explanation: http://www.latimes.com/features/food/la-fo-master-class-thomas-keller-20110428,0,3142391.htmlstory

When properly seasoned, food shouldn't taste salty. Adding salt at the end of cooking almost invariably makes it taste salty, rather than seasoning and intensifying flavors.

Salt is used to draw the moisture out of sweating vegetables.And it softens them faster.

Certainly many foods need salt added during cooking, for convenience sake if nothing else. But meats I think often benefit most from salt added just before putting them in your mouth. Salt added during cooking just soaks in and makes them taste SALTY like a cheap hotdog or a can of Spam. The flavor just seems burned out, and the extreme saltiness makes me very thirsty. If I'm cooking a nice steak on the grill and cut it up and sprinkle the salt on at the last moment, it really brings out the flavor without using all that much salt. It is almost like I'm salting my taste buds rather than salting the meat, and changing their perception of the flavor rather than altering the composition of the food.

Also consider that the process of osmosis tends to draw water from the less salty interior of meat the salted exterior, which would tend to remove juiciness. Obviously at some point the salt does penetrate inwards as well.