Salt isn't absolutely required to make bread or pizza dough, but without it, breads simply taste flat—even sweet breads. That's reason enough to add it.
But there's more: salt also strengthens and tightens the gluten and regulates the activity of yeast. Without any salt, some breads can rise unpredictably. Some sources say that since salt toughens the gluten, it's best to add it at the end of kneading, to make the chore easier. Others say that it should be added at the beginning, for better distribution. Personally, I haven't seen much difference, so I'll leave that decision up to you.
Whether kosher salt, sea salt, or regular table salt, all salt is chemically the same (though some fancier sea salts do carry a small percentage of extra minerals). The real difference in how salt behaves is in the structure and size of the crystals, and some cases, added ingredients.
As far as add-ins, recently I've seen an item marketed as Bread Salt that looks very much like the salt marketed under the brand name Real Salt. It's mostly pink with flecks of darker reds and looks pretty in a container. This salt is said to contain extra minerals which may be better for you or for your yeasty friends. I've used it and again, I can't say that I noticed a difference in the finished product.
Non-branded pink, red, black, gray, white, and smoked salts are also available, but most of these are relatively expensive and are intended as finishing salts—that is, you sprinkle them on finished products where they'll have the most impact. They usually have larger crystals that don't dissolve until they hit your tongue. Many of these specialty salts are sea salts harvested from evaporated ocean water.
For everyday use, most people rely on table salt or kosher salt, both of which usually come from rock salt. That is, they're mined from the earth rather than harvested from salt water.
Table salt is a fine-grained salt, and comes either iodized or not. Iodine is necessary for proper thyroid function, but most people get plenty through other sources, so it might not be necessary to use iodized salt. Table salt also usually contains anti-caking ingredients to keep the salt from clumping.
Canning and pickling salt is much like table salt in form, though its grains are smaller so it dissolves faster. More importantly, it doesn't contain the iodine or the anti-caking ingredients which turn the pickles dark or cloud the pickling mixture. You can use this instead of table salt, although it does tend to clump a bit if it's left undisturbed for long enough.
Kosher salt has larger, more uneven grains than table salt, but not all kosher salt is the same. Morton's, for example, contains anti-caking agents while Diamond Crystal (the other major brand) does not.
The bigger difference between salts, however, is their relative densities—that is, how much equivalent volumes of different salts weigh. You see, the saltiness of a given amount of salt is proportional to its weight, not its volume. Why does this matter?
It means that unless you are weighing all of your ingredients, you can't substitute one type of salt for another. For instance, Diamond Crystal salt is about half as dense as table salt. So if a recipe calls for Diamond Crystal and you substitute table salt, your food will end up about twice as salty!
Using a gram scale, I weighed out teaspoons full of various types of salt and got the following:
- Table Salt: 7 grams per teaspoon
- Morton's Kosher Salt: 6 grams per teaspoon
- Diamon Crystal Kosher Salt: 4 grams per teaspoon
- Large Flake Sea Salt: 3 grams per teaspoon
Of course, measuring inaccuracies and scale rounding errors could mean your weights for the same salts could be a little different than mine, but in general, if using Diamond Crystal salt, you'll need about twice as much volumewise to get the same salting level as table salt.
There's one last salt to consider when making bread: Sour Salt. This isn't a salt at all, but is actually citric acid crystals. It's useful for adding a bit of sourness to your bread—really nice in rye.