Here’s a great article why Zone 2 training is critical. It’s detailed and a good read. Enjoy and share with your fellow endurance athletes.
Teach your body to use fat as fuel. This is important to conserve your carbohydrate stores for later in races when you need to up the intensity (short distance triathlons) or for finishing a long distance race strong by still having carbohydrate stores for fuel.
Develop the muscle fibers that are responsible clearing lactate. The ability to clear lactate is critical to performance.