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.