IMO, the heat sink keeps the temperature more stable, i.e. might add a 10 minute delay before you notice a change if you make an adjustment or your coals finally burn up. Water is about the same but it forces the temp to stay closer to the boiling point (~212F, depending on your elevation). Waterless with no heatsink, what you see is what you get. If I open the lid & the fire's fed, the temp will bump up, but there's no heatsink other than what you're cooking, and it'll get right back into line very soon after you put the lid back on.
It's all fire management - you'll figure it out soon enough. Just don't sweat it and keep it easy, you have to try pretty hard to mess anything up.