Sealcoat helps prevent oxidation from the sun and water penetration.
Asphalt gradually deteriorates, and as it does, it cracks and pits. When water gets into those pits and cracks, and swells and shrinks with freezes and thaws, the cracks and pits get worse, dirt gets into them, and then plants start to grow, splitting things more.
It also helps rejuvenate the asphalt.
Sealing your driveway prevents the water getting in to do the freeze and thaw routine and protects your investment.