Here we will examine the tradition of bread & salt.
A person can live on bread and salt. The body needs a certain amount of salt. Bread is fairly nutritious if it’s not baked by Wonder and their ilk. The Romans used to pay the troops in salt- hence salary. Salt was always considered a valuable essential and it translated into money in many cultures.
As to why the importance of salt, even over water, and even in desert cultures? In one word: Electrolytes.
You sweat, you lose sodium. You pee, you lose sodium. Lack of sodium screws up your potassium levels. Screwed up potassium levels can cause nausea, muscle cramps, seizures, and heart arrhythmia with further results up to and including death.
Lose enough salt without replacing it, and even water doesn’t keep you alive. That’s why there have been salt tablets in military and expeditionary kit for decades, if not centuries. It’s why the Roman legions often had vinegar in their canteens, not water. It’s why the waterbearers carry Gatorade, pretzels, and pickles, as well as water.
The chemistry wasn’t known in the Middle Ages, but the result was. Now we simply know ‘why’ as well as ‘what’.
To offer someone salt doesn’t merely offer them the stuff they need to be alive, it offers them the stuff they need to be healthy. So, offering salt was hospitality beyond what offering water was.