Sodium Bicarbonate or Baking Soda was used in the past (and still is today) as an antacid, used to settle stomach problems such as indigestion and heartburn. When the Digestive Biscuit was first invented in 1892 by a gentleman named Alexander Grant who worked for a company called McVitie and Price, they contained a large quantity of baking soda, as such they were believed to also aid in digestion and were dubbed Digestive Biscuits.
However, in order for baking soda to work as a digestive aid it was consumed after being dissolved in water. When the biscuits are baked it alters their chemical structure, they lose the carbon dioxide that provides the qualities that neutralise stomach acid and removes any antacid or digestive aiding properties that the baking soda added to the biscuit.