Our brains are wired to consider food more palatable if the dedicated taste receptors (one of the six basic tastes) for salt are triggered. And in a non-urban world, salt/sodium is a valuable nutrient; the fact we might have it too readily available in the developed world doesn't change that we would DIE on a zero-sodium diet (mind that animal products like meat aren't zero-sodium, and that herbivores tend to love licking salt where they find it!). So our brains have a good evolutionary reason to like salt.
Also, salt actively interferes (negatively) with another basic taste receptor - the receptor for bitterness (which can mean poison both in nature and in the developed world, or at least something we have no use for, eg an alkali). Most aromatic food (think green vegetables or spices) is bitter, and salt both attenuates that perception and balances it making the combined food still desirable for our brain. You end up with an even more palatable food since you can use an aromatic (desirable) and shut out the bitterness (not always desirable) response.
Umami is a different basic taste, probably related to the presence of protein (glutamate, inosinate, guanylate... trigger it - these are amino acids or salts thereof, and an indicator of easily-digestible protein presence).