Earlier harvest reduces crop losses from inclement weather, wind, stalk rot, and insect damage.
Early harvest conditions are usually more favorable during high-moisture corn harvest than during dry corn harvest. 
Harvesting both high-moisture corn and dry corn spreads out the use of your harvesting equipment and labor.
Early harvest also allows for additional options with your farmland, such as allowing earlier stalk grazing or baling.
Corn is considered mature when it yields the maximum quantity of dry matter near the 30% moisture range, depending on the hybrid.