DNA encodes proteins. Enzymes are a type of protein that act as catalysts in your bodies metabolism. If you have a mutation that causes an enzyme to either not work or exist, you may need to replace it in order to properly digest food, eliminate toxins, or carry out basic metabolic activity.

A very common example is Lactaid. If you are lactose intolerant, you can take a pill containing lactase, the enzyme that digests lactose, so that you eat lactose containing foods without issue.