Systemic digestive enzymes are enzymes taken on empty stomach in order to get absorbed to the blood stream and get to all the systems of our body where they may have beneficial effect on our health.
In young age we have enough of our own digestive enzymes. They properly digest foods and do the systemic job as well. When we get older we don't have good enough production of those enzymes. It makes sense to supplement them in order to avoid health problems related to their deficiency.