I'm the original poster -- here is my real-world use case where a null edit is
required.
My wiki has a custom parser tag, (say) <foobar>. Many articles include it in their
wikitext.
We modified the code for <foobar> to add a feature: autocategorizing. It
automatically categorizes any article that contains it (say, in "Category:Articles
containing the foobar tag").
After this code change is deployed, if you visit an article Blat that previously contained
<foobar>, you will see it is properly categorized at the bottom. However, if you
visit the category page for "Articles containing the foobar tag", it is missing
Blat (and tons of other articles).
If you perform a null edit on Blat, it then shows up on the category page. Purging Blat
(action=purge) does not have this effect.
DanB