Sealcoating doesn't just make your asphalt black, it's necessary to keep your driveway from deteriorating. When the liquid asphalt that's holding the aggregate together gets oxidized, your asphalt becomes brittle and is prone to cracking. Sealer helps protect your driveway from the effects of the environment, think of it as a sunscreen for your asphalt.
How often should asphalt be sealcoated? The short answer is every other year, but there are exceptions. When a driveway is new, after it has cured, it should probably be sealed the first two years. The first application, generally doesn't cover as well as the coats that follow. After the first two years, every other year should suffice. Ideally, you would like for the sealer to wear off between applications. When a surface is sealed too much, the sealer will build up and can begin to crack.
If you have an asphalt driveway, it should have the texture of asphalt. If it looks and feels like a black skating rink, you've sealed way too much.