A grind on demand grinder grinds the coffee only when it is needed and will grind for a set amount of time to give the correct dose. Why is this so important? Grinding fresh on demand is so important because you need fresh coffee for espresso drinks. Espresso contains crema which is a foam made up off gases and oils. Fresh coffee contains more gases and these gases are needed to create the crema.
Once coffee is ground it starts releasing gases very quickly. Minutes will make a difference. The fresher the coffee the more gases are contained within the ground coffee. The gases are forced out of the ground coffee and into the liquid by the pressure. The oils in the coffee then form a layer around the gas bubbles creating a foam which we call crema (cream in Italian). Fresher coffee creates more crema.