Let's I save a page [[A]] with {{subst:foobar}} the template for which doesn't exist yet. Then I create the template, possibly to contain something offensive. Then you accidentally vandalize page [[A]] when attempting to do a null (or completely unrelated) edit.
Back in the day, we used to use null edits intentionally to update pages after templates changed. Now we have the job queue to do that for us, but it still works as far as I know.