Whenever you put salt on food, you never say that was great salt, but rather that was great food and it tasted great. Likewise, whenever you are salt to a group of people, people don't go away saying what a great person you are, but rather what a great group or meeting that was.
The simple fact is that salt makes people feel better about life. It could also be described as influence. A person of influence (positive of course) can really make a difference to those around them. Leaders should be such people. There are many sources of influence in this world, many of which lead people down the wrong path.
Natural and effective leaders are like salt to people in just the same way salt is to food. Their influence leaves followers equipped, empowered and inspired to go further, reach higher and overcome obstacles, yet very often followers aren't even aware of the salt cellar being shaken.