Consciousness does work. My being consciously aware of various things motivates me to react in various ways, to do things in response. The zombie scenario claims that consciousness has been removed, and everything else left the same, with "everything else" including the work that consciousness does getting done. But that's impossible; if consciousness is removed, what it does won't get done unless something else does it instead (and the zombie scenario is certainly not supposed to be adding any substitutes).
To put it another way, if in a physical duplicate world the work consciousness does is still getting done, something physical must be doing the work in that world. But if it's a physical duplicate world, whatever it is that is doing the work in that world is also present in this world. But in this world consciousness is doing the work. So whatever that physical thing that's doing the work is must, after all, just be consciousness.
This didn't end up being as short as I expected. Still shorter than the last two posts on the topic, I suppose.