Certain fluids leaking within security equipment could compromise its effectiveness. I can understand why they might insist upon a sealed bag
Now you mention it, that does make some sense.
the world seemed to manage OK without putting liquids in sealed bags prior to 2006 so obviously this is not a reason to have sealed bags.
I can imagine when the "liquids in bags" rule was introduced someone thought - we're going to be scanning a lot of these bags lets have a rule that they must be sealed.