As you may know, Chrome and Firefox already have built-in spell checkers that work as you type. So why bother with AtD? Simple: the latter has a lot more features, including context-sensitive checking and the aforementioned style suggestions.
Bottom line: if you’re not too swift with the arranging and spelling of words, at least let After the Deadline do a little proofreading before you click Send. It’s free, so what do you have to lose?