The world's oldest continually valid food and drink regulation. The reason for the law was to assure that valuable bread cereals would be used for beer brewing.
The actual Purity Law is a result of the Bavarian Duke, Wilhelm IV. He instated the law at the Ingolstadt Parliament in 1516. The reason to institute such a law was mainly the quality of "Beers" at that time. At that time, spices used were fruits, herbs, weeds such as anis, myrtle, oak leaves, ivy (poisonous), along with the poisonous seeds of herbstzeitlosen, raspberries, elderberry, caraway, lavender, dandelion, bay leaves, balm, mint, nutmeg, cherry leaves, plums, rose leaves, rosemary, wild rosemary, schluessel flowers, juniper berries, and lemon were normally used for brewing. Thereby, there were certain reasons for these uses. As an example, some ingredients were used as a substitute for hops, others for the intoxicating effect, others to extend the shelf life. It is easy to see that beers at that time had little to do with what we know today as beer.
Thank goodness for the German Purity Law of 1516. If not beer
would taste like crap and we would die of food poisoning.