Why is it a good reason to read through an application before starting to fill it out

Because this way you can ensure yourself that you answer everything they ask you

I agree with the first commenter : ^) it's like science. You always read the instructions before you start an experiment so you know what you're going to do and don't screw something up. With an application you're reading it so you can make sure of what you're going to answer instead of filling it out and possibly your story happens to not sound very straight - as in they will say hm somethings off; they're faking it and we don't want to hire them.