Sodium is one of the body's electrolytes (minerals that the body needs in relatively large amounts). It helps maintain the body's water balance along with chloride and potassium. Additionally, it contributes to proper muscle contraction and nerve impulse conduction. You can see why it's a major mineral.
A majority of sodium in the body is found in the blood and the fluid around cells and therefore plays a major role in maintaining blood pressure. The body gets sodium through food and drink and loses it primarily in sweat and urine. So the kidneys play a key role in sodium balance.
As people age, the body is less able to maintain fluid and sodium balance. This may be due to obtaining less water because of bodily changes from aging (decreased thirst sensitivity, kidney function, decreased bodily fluid), physical or cognitive issues (immobility, dementia), or certain medications. There may also be too much fluid in older individuals due to certain disorders such as heart failure, liver disease and kidney disease, as well as from taking diuretics (water pills).
The good thing about cell salts is they are not supplements but microdoses of the essential elements and nutrients. Therefore there isn't the same risk of taking too much. The salts trigger the body to do a check of what is going on and course correct if necessary. This is why the natrum (homeopathic word for sodium) salts #9, #10, and #11 are used by individuals for water balance and digestion.