1. What advantage do these get you over the traditional SI firebox?
I don't have one (yet), so I can only answer the first question.
If you want to cold smoke then you can use your SI smoker as just the container and use one of these smoke producing devices to provide the smoke. This keeps the temperatures significantly lower and means you don't have to get the cold-smoke plate accessory.
I actually have the cold-smoke plate and have done cheese a couple times, ham once, and bacon once for cold-smoking. What this looks like is using chips of wood instead of chunks, putting in the cold-smoke plate, using the drip tray to hold ice on top of the cold-smoke plate, and then cycling the power on my #2 for 15 minutes on and 30-40 minutes off multiple times. The cycling is to get the wood smoking, but to not keep applying heat in the smoker as I don't want to significantly raise the temperature in the smoker. It works, but it is admittedly a bit of a hassle.