Why is salt sprinkled on top of soft pretzels?
IMO the distinctive flavor of a soft pretzel isn't a bland lack of flavor, but rather a slightly bitter, slightly chemical flavor that comes from the lye (or sometimes baking soda) solution in which pretzels are briefly cooked before baking. The lye solution does at least two things for pretzels:


It raises the pH of the pretzel's skin, which in turn makes the pretzel brown nicely as it bakes.
Imparts that slightly bitter flavor.


I don't think the salt used on pretzels is there to make up for a lack of salt in the dough. You can make pretzels that have plenty of flavor even without salt. I think the salt is there mainly because people like salt, because it adds a pleasing crunch, and because it's traditional -- it's part of what people (or at least Americans) expect from the pretzel experience.

It's worth mentioning that the proper salt for topping a soft pretzel is not kosher salt or rock salt, but a white, soft salt like Cargill Pretzel M Salt. Kosher salt will work if it's all you've got, but it's not what you'll find on a soft pretzel bought at a ball park or from a street vendor.