Repairs, especially on laptops, can be pricey. A logic board replacement can go to $ 1000, and a screen replacement is at least $ 300. At least if anything goes wrong, and you'd need a more expensive repair, it's covered.
Plus even if you don't end up needing it for any repairs, you can always use their phone support too. And if you want to upgrade your MacBook Air to a newer model in like 2 years or so, you will get much better price for your laptop than if it didn't come with AppleCare.
So good for either case. And don't forget it gives global coverage too - so if you'll travel in Europe, Asia or somewhere you can still get support for it.