an unusual use of the word justify

Every time you use your word processor you use justify in exactly that sense.

Yeahbut®, why the word justify? You don't have to justify why you want the text lined up in a particular way--you just do it. You set it. You align it. You make it even. You make it match. Etc., etc., etc.
If your boss questions your judgment, you might have to justify why you've set it a certain way, but I still don't see why the word justify came to be used in reference to lining up text.