Some plants require sodium to aid metabolism, albeit in minute quantities. The need for sodium for many animals is so strong as to comprise an appealing taste sensation in of itself. In mammals, sodium facilitates nerve function, helps regulate blood quality and maintain fluid balance.
It is difficult in modern diets to not get enough sodium, as it features in almost all processed foods. The health problem with sodium tends to be getting too much. In natural foods, seaweed and spinach supply ample sodium.