You can throw chunks of wood on top of Charcoal in any grill.. The problem is the first thing that turns to the vapor is the gas in the wood. If there's enough oxygen it'll burn as a flame, if not it'll create a thick gray bitter acrid smoke. This is the nasty smoke that burns your eyes, sticks to your clothes and hair, and makes you smell like a forest fire. The cast iron pot runs the gas back down thru the burning charcoal to clean it up.. sorta like an after-burner. The cold smoker creates just enough vacuum to make the wood smolder, this keeps it from getting so hot the gas leaves without being burned off. If you go to the old school BBQ joints, they burn wood until the flame (gas) is gone and then throw the charcoal into the grill to capture the secondary/blue smoke.
This secondary.. after the "gas" vapor is much smoother and lacks the bitter nasty acrid flavor profiles the first smoke/vapor has..
Like everthing, there are different levels of preparation that create results with different qualities..