Look it up!

In a Language Log post on "ear spellings", Mark Liberman observes:

A reflection on why ear spellings should be so likely for this word. If you've heard the word, you probably know how to use it in sentences, but if you haven't seen it in print (or don't remember having seen it in print, or didn't realize that the spelling "voilà" represented this particular word), you're in trouble. People tell you to look up words if you don't know their spellings, but where do you look in this case? If you don't know French, or don't recognize the French origin of the word, what would possess you to look under VOI in a dictionary, especially if your pronunciation of the word begins with /w/ (I think this is the most common current pronunciation, at least for people who aren't "putting on", or at least approximating, French)? So you spell it "the way it sounds".

I have to say, I've never quite understood that injunction. Having a sister who's educated, literate, and unable to spell made me familiar with the problem, which can be summed up as: How can I look up how to spell it if I don't know how to spell it?

In other words, what good is a dictionary, which lists words according to how they're spelled, for someone who doesn't know how to spell a word - especially if they're faltering at the word's beginning?