Problem:

A few friends of mine told me that salt provides zero nutritional value to us, and in fact can harm our bodies. Now, these guys are medical students, and being an engineering student myself, I decided not to argue with them. The rest of this question assumes that this fact is true, so if it's not, you can just go ahead and call me out now...
So here's my understanding of things: we 'like' doing things because of our instincts, which have slowly become refined over millions of years. For example, I 'like' eating foods with fat in it because my instinct compels me to do so. Fat is 'good' for my body, since it provides a lot of energy (obesity problems aside).

So are there certain things, such as eating salt, that are not in fact beneficial in any way, and we only do these things because we were trained to as children? This is the only thing I could come up with, but it's not a very satisfying explanation for a few reasons. First, I think humans have been eating salt for a long time. This would mean that most likely, it is actually our 'instinct' to eat salt. Also, salt is eaten inevery culture today, which has the same implication.

Please provide some better explanation for these statements?