Salt shows up in nearly every recipe, which prompted my daughter to wonder why. On Food and Cooking doesn't have any relevant information, although the list under Salt in the index reinforces the ubiquity of salt in preparing food: in baking, in boiling water, in cheesemaking, curing of meat by, egg cookery, in egg foam, in freezing ice cream, in milk curdling, preservation of fruits and vegetables by, and in sauces.
Joy of Cooking is more help, starting with the observation that "the interplay of salt and water is essential to life itself", which explains why humans eat salt, and why we appreciate the flavor. What is startling is how often salt interacts with other foods in ways that go way beyond making them taste salty. Salt preserves all manner of food, from oysters to pickles to meat; the salt draws out the moisture in the food and creates a difficult environment for micro-organisms that would otherwise spoil the food. Because it draws moisture out of foods, it firms vegetables and helps maintain the shape of grain in boiling water. On the down side, it toughens eggs (oops - good to know when I scramble them: I'll try adding salt at the end, after the cooking is done), and too much of it can make for leaden lumps of bread since it inhibits rising.
So the answer seems to be twofold. The presence of salt on steaks and in eggs can clearly be attributed to the human preference for salt; a dash of salt will enhance the flavors of most any food because the tongue is presumably evolved to appreciate salt. And salt's ability to draw water out of food seems to account for its presence in most of the rest of the recipes. That seems like a pretty prosaic answer for such an amazing ingredient.