Sometimes, you’re just typing an e-mail or note on your iPhone, and you realize that you have no idea how to spell the next word you want to use. It could be genuine ignorance, it could be a brain fart, but the person on the other end isn’t going to care why; they’ll just notice the mistake.
Spell Checker wants to help you out. It accesses your onboard dictionary to keep you from looking dumb. And because it uses the built-in resources, it even works offline.
You know, in case you’re writing an e-mail in a cave that you would want to send after you left the cave. It could happen.