It's boringly simple. Their simple ingredient and plain content mean that the biscuits were thought to be and aid to digestion, and were marketed as such. Other digestive trivia: The ovens in which the biscuits are baked are 300 feet long; digestives are produced at a rate of 450 biscuits per second; if you laid out the amount of chocolate digestives eaten each year in Britain it would stretch twice round the world.