Funny enough, though their jobs would be the easiest to replace, they are "important" in that they can be held legally responsible for business decisions (Responsible Corporate Officer doctrine). That's partly why they get tens of millions of dollars - so they can take the fall when shit hits the fan (either they fuck the shareholders or majorly break the law).

This is known as the "Scapegoating Premium" - you are basically paid for the risk of taking on a very public, high liability/visibility position. The corporation pays their legal fees and has insurance to help with this, but it's compensation for the possibility of legal and reputational damage.