I think whenever someone opens your case, the "last updated timestamp" updates.
So usually when you submit the DS 5535, there'll be a bunch of updates as they're doing something with your case. Every update thereafter is seen as a good sign that someone is actively working on your case, rather than it sitting on some queue, waiting to be touched.
Of course this is all speculation as I don't know anything about what actually goes on, so YMMV