Take Everything With a Grain of Salt: A Brief History of Salt (Kurlansky's) + A Conversion Chart
Homer and Plato both described salt as divine, although Lot's wife wouldn't have thought so, having been transformed into it. According to Tacitus, certain Germanic tribes believed prayers were heeded somewhat more if they were offered in salt mines; during China's Cultural Revolution, Chiang Kai-shek's local headquarters were turned into a salt museum. (All this, and far more, can be found within the pages of Mark Kurlansky's excellent 2002 book Salt: A World History.) Salt was once so valued as to be traded as currency--the word salary comes from it--as it preserved food in the long warm history before refrigeration.
Morton's Salt Morton's 1917 ad
Although we don't need salt with quite the same desperation as we once did, salt is enormously valuable in the cooking and preparation of food, quite aside from its preservation qualities. More than any other ingredient, it's responsible for much of the flavor we get out of food. Forget the salt in a recipe and you'll be astonished at how bland it is, how the flavors don't accelerate, how many of the nuances are lost. And this goes for desserts too: add a pinch of salt to a bowlful of chocolate sauce and you can suddenly taste twice as much of the chocolate. That said, salt gets a bad rap, and often deservedly so. Read the labels of most prepared foods, and you'll find enough salt to preserve the food in the package, the rest of your dinner, and yourself too, probably for some time.
So it's important to take information about salt, well, with a grain of it. (Taking something with a grain of salt--a cautionary tale--dates back to Pliny the Elder, who in turn took it from Pompey: Pompey's antidote against poison was "to be taken fasting, a grain of salt being added.") And there's not only quite a lot of information about salt available, but quite a lot of salt. Maldon salt, kosher salt, table salt, sea salt from Wales and Normandy and Hawaii, flavored salts, rock salt, etc. Not only do many of these salts look and taste differently, but they behave differently as well.
Shirley Corriher, in her lastest book Bakewise, has an invaluable section on salt towards the end of the book's 500-plus pages. In it, she not only describes the differences in composition and structure, but provides a handy equation for cooks. This is because, as Corriher writes, "not only do these different forms of salt dissolve, mix, and adhere differently, a given volume, say 1 tablespoon, contains a different weight of salt for each form." Makes sense when you think about it--but who thinks about this? Well, Corriher does, thankfully. Write this down: 1 tablespoon of granular salt = 1 1/2 tablespoons of Morton's kosher salt = 2 tablespoons of Diamond Crystal kosher salt. If you've ever found yourself substituting kosher salt (the default salt for many chefs and home cooks) for regular table salt and found that things didn't quite turn out right, now you know why.
As for why chefs use kosher salt, many like its texture and mild taste. Other chefs like sea salt, of one sort or another, often depending on regional loyalty. Salt is also often used as a finishing component, sprinkled atop dishes in much the same way freshly-ground pepper is, added at the end like a squeeze of lemon or a splash of olive oil or vinegar, to brighten the flavor. Maldon is often the best for this, as it has a particularly bright flavor and the flakes are large and quite pretty. But then you can take all this with a grain of salt too.